Perhaps someone at BSF pre-empted your thoughts BTDT. The (infamous) model 65 I have has the power restriction symbol 'F in a pentagon' on the side so it was originally produced as a low powered (7j) version. During its lifetime someone has fitted a stronger spring to raise the power lovel.

The BSF 55/60/70 are quite lively to shoot and require a loose hold to gain the best from them, similar in style to the standard 'slow action' of the 'Sport. Interesting enough the fitting of a shorter, stronger (?) Macarri spring to a 'Sport quicked the action and altered the best holding stance.

ATB
Ian