Recently I got a German translation of Casanova's memoirs. It's a thick book, but it looks, as if it isn't complete version. I didn't find any note about a Cologne lover of Casanova, which should belong to the year 1759 according some research, which I found in the web.
Wiki tells, that a complete version was published in 1960 by the publisher Brockhaus ... in 12 books.

The card divination scene of 1765 is included. The Russian girl (Zaira) should have been "nearly 14 years" old and Casanova bought her from her parents, a farmer family close to St. Petersburg. The relation took about a half year, it was finished in autumn 1765 with the card divination scene, when Casanova found it too dangerous to live with her (she had thrown a bottle on him cause of jealousy).

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In detail ...

Casanova left Riga at 15 December 1764, and reached Petersburg around 21/22 December. Then he spend some some time (some descriptions) and bought the girl (? begin of spring ? 100 Rubel). She accompanied him on a journey to Moskau (May 1765), the whole took a week to get there, he spend 8 days there, and then returned (another week). He spend a lot of time with her, and she accompanied him on public events.

Around end of August (the news came, that emperor Franz I had died at 18 August) he observed reactions. He planned to travel back to Western Europe at begin of autumn, but he had opportunity to talk to Zar Katharina. So he stayed longer. The text is not really clear, but I think, that the divination scene took place October/November 1765. End of January 1766 he was already some longer time in Warschau, Casanova reports a prophetic dream for this time (which announced a duel in Poland).

The interesting question might be, where and when the 14 years old girl might have had opportunity to learn how to read cards (with 25 cards).
The most logical answer seems to be, that she learned it during her time with Casanova. A relative poor background as a farmer's daughter doesn't promise that she learned it before. Or? Perhaps a farmer family "around St. Petersburg" had chances to have some some Western relations?

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There's a strange system, mostly called "Runes" and in one case "Slavonic Runes" spread in Russia ...

In the Museum ...
http://a.trionfi.eu/WWPCM/decks02/d00504/d00504.htm
http://a.trionfi.eu/WWPCM/decks02/d00302/d00302.htm
http://a.trionfi.eu/WWPCM/decks02/d00511/d00511.htm
http://a.trionfi.eu/WWPCM/decks02/d00854/d00854.htm
http://a.trionfi.eu/WWPCM/decks07/d05454/d05454.htm
http://a.trionfi.eu/WWPCM/decks/d00200/d00200.htm
http://a.trionfi.eu/WWPCM/decks/d00197/d00197.htm
http://a.trionfi.eu/WWPCM/decks06/d04036/d04036.htm
http://a.trionfi.eu/WWPCM/decks05/d02313/d02313.htm
... and some more

... all with 25 cards. The decks are all of a younger date (plausibly produced since c. the year 2000).

Some Western producers seem to have imitated it. Or did the many Russian productions imitate a Western interest with some enthusiasm?
Maybe that's just a lucky accident with the 25 cards, once in the spread of Zaira and then those in the "modern" Runes deck.

In the modern esoteric times Runes appeared on the book market in the 1980s.


http://www.bewitchingways.com/runes/runes.htm

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Further there is a deck type (25 cards, mostly 36), in which each card contains 4 picture-halves (so totally 50 or 72 pictures). So like this ...

http://a.trionfi.eu/WWPCM/decks02/d00854/d0085418.jpg

I remember to have bought a version of the "horoscope of Seni" in the 1970s, which was a 36 cards version, using 72 astrological signs (star pictures mostly). The accompanying text (which contained the divination text, containing 4x72 = 288 divination; each fitting picture could have 4 positions left turned, right turned, upright and on the head) in the style of a lot book indicated (without convincing proof ; the story is told, that it was detected by a German officer in 1870/71 at a French castle), that Wallenstein's astrologer Seni invented it (which would make it a 17th century invention) and left it itself at the castle.

Another possibility, how it might have been precisely 25 cards in the oracle deck of Zaira ... if the game type is indeed from 17th century. My text has no production date in the impressum, possibly to add to the impression of age and it is printed in old German fracture with possibly the same wish.

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The one sentence, which speaks of 25 cards, sounds in the German Casanova text indeed, as if there were only 25 cards, and not more. Though this happens not in a sure way, so that one cannot rely on it.

"Um mir mein Verbrechen zu beweisen, zeigte sie mir 25 Karten, aus denen sie auf dem Tisch ein Viereck gebildet hatte."

The question that occurs to me is this: if it was a deck of 25 cards, and all 25 were laid down, what would distinguish one reading from another? I can think of two possibilities:

(a) only the first five were laid down face up, and the others were only used in case the card above it made no sense in the context of the other cards, the question being asked, and the circumstances of the person being asked about.

This is a standard method, found in Etteilla's 3rd Cahier and also "Julia Orsini", c. 1838, although the usual procedure was 4 rows of 5 instead of 5 rows.

(b) all the cards were laid down face up, and the cards in the lower rows modified what was said by the card in the top row.

In this case, it would be possible to "see" whatever one wanted in the cards, consciously or unconsciously. If one were the jealous type, one could always see reasons for jealousy there, every time, one way or another. There is, to be sure, some randomness in the layout of the cards, but with enough ambiguity in the meaning of each card, and enough picking and choosing, this randomness could be overcome easily.

In the case of b, I don't think Casanova would have been impressed, over time, with the girl's divination skills. Even in the case of a, it would be questionable, since she would simply be going to the next row, etc. for confirmation of her fears. It would be more impressive, although requiring more imagination, if she used a larger deck--36, 52, 56, or 78 cards. She could still find confirmation of her jealousy, if she was quick and inventive enough--after all, she has 5 chances--but with more randomnness, it would appear more the work of "fate" or some guardian spirit--and hence more likely to be true--if she used a sample of a larger number.