@Veshv
The contrasts and their personality differences are quite interesting, especially in how it showed in this gamble. Hachina seemed to be more focused on pride and being upright, accepting the gamble out of pride when he didn't need to, not caring about the opponent's lack of violence, and trying to be mindful of the bookstore owner's property by saving the book. While Baku plays a lot more savagely and selfishly, setting up all sorts of tricks like suggesting the page flipping game because he knew he could do it perfectly, or suggesting the book game because he already knew the prices of various rare books, and also casually ruining someone else's property.
Hachina didn't even try to oppose Baku's suggestion on the rules of the game, and just let his opponent gain an advantage for fr. And his strategy in the end, choosing the gravure book, was more based on him being lucky, because as you said, he didn't try to oppose his opponent's strategy at all. I can understand why Baku said something along the lines of 'that noble kind of thinking pisses me off', it felt like Hachina wasn't playing 'seriously' at all, because he was limiting himself by upholding all sorts of personal rules and ideas.
It seems like at this point in time, Hachina was a lot less competent a gambler compared to current Hachina. And I imagine that Souichi is even more OP than that. I wonder if he picked up those skills from Baku?
Another small contrast I noticed, they probably have opposite handedness, Hachina used his left hand for the page counting game while Baku used his right.
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edit & random thought:
It would be really funny and interesting if Baku proposes the same game from the past and tries to repeat the same strategy on the current Hachina.