I'd be very supprised if you could hold even an old crabtree C50 closed against a fault, certainly the old loadmasters(and arguably being a worse product, being type 4) that were around at the time were not subceptable to that.according to the original manufacturers data sheets from the 70s. I do know that C50s were hydralic in nature (think john ward took one apart on you tube) so how this affects things I do not know.
Imagine the effect is something like that if a breaker is only just on the point of tripping, then if the toggle is held up, the little bit of mechanical resistance through the mechanism can turn a marginal trip, into a marginal non-trip, outside this marginal window though the extra mechanical resistance is not sufficent to stop the breaker unlatching independant of the mechanism, so the characteristics have changed slightly but still within the allowable tolerance for the device. I've seen this happen on at least one occasion (and heard - you can hear the breaker nearly trip, but not quite get there). Board change in a bakery, three phase dough hook on a 20A Type 2 before, due to someone not ordering it correctly it ended up on a 20A type B which it threw out everytime it tried to start, bit of a mad scramble at silly O clock and we managed to turn up a 20A type C, which would let it start about half the time, or every time if someone held against it. Now type 2 has a magnetic tripping range of somewhere between 4 and 7 times, type B between 3 and 5, type C between 5 and 10. Now I can only assume that the old type type 2 happened to be something like 6 times, but the newer breakers were intended to hit the more sensitive part of the tripping range. It ended up going back on a type D (10-20)