I keep finding dorman smith and merlin gerin boards with hager MCBs stuck in them (and occasionly wylex and proteus as well), I hate it, its just lazyness, can't be bothered to order the right breaker..If manufactures original parts are available I doubt any competent electrician would choose to fit other manufactures parts. However we sometimes have to make a considered judgment over what is best for the customer..
If its older equipment then I have done it on a time or two, but usually can find one thats been removed from a board change thats right for the board