Hi,
According to our US sister site, .net, the 7A carbs were fitted to the early K1 (like mine). These bikes were made in the original 750 factory, before they moved to the "new factory" this I believe is when they changed to the 657. However, it is my understanding that pretty much all the variants of these carbs are down to the internal configuration. The castings look identical. So I suggest you check the main jet emulsion tube to see if it matches your other 3 carbs (there are subtle differences, spacing and hole sizes and height on the tube), the jet needle height and of course the jet sizes. It might be worth a check on the air bypass restrictor for the main jet emulsion tube too. (the little brass air jet pressed into the body in the intake bell)
Have fun.