ONe thing i'd check is that the centre section for 125 250 and 465/500 are same part number, otherwise the box is same as my rm250,465,500, all 3 are the same, just might be that the hole for the carb junction to box might be different,never measured that or swapped bike to bike.
you could also ask them to give you the i.d of the hole, I have measured mine....its..... 61mm and carb is 62mm...38mm original square slide Mikuni and both off RM465Z identical to the box and carb on 500d parked next to it.
Kt.