Our world revolves around money. The accordion world is no exception. Every accordion brand makes C-griff, B-griff, C-griff with C on the third row (Finnish system), B-griff with C on the second row (was popular in Belgium), Balkan B-griff with 6 rows, ... anything the customer wants. And for the bass side it's the same: C-griff, B-griff, low notes at the top or low notes at the bottom. And of course any keyboard side can be combined with any bass side as the internal construction of the body and reed blocks does not change when the keyboard changes so they all fit together.
So much depends on coincidence. I keep on saying that I (and my wife too) should have started with CBA at least 10 years earlier than I did, but at that time I was living in Belgium in an area where most people play B-griff so I would probably have ended up playing B-griff. As it happened, after moving to the Netherlands and seeing few CBA players, but all with C-griff, we ended up starting on C-griff. This was an expensive choice because I have seen many excellent used accordions for sale that were all B-griff, and at the time no C-griff so we ended up with the expensive choice of buying new. Later the situation started improving.
A used Akko or Jupiter or Pigini should be relatively easy to find in B-griff, but with that C-griff bass side, not a chance. However, the good news is that when you master your current instrument a Russian B-griff should not be too hard to learn as it is just your bass side rotated by 180 degrees (not mirrored which is a harder conversion to learn).