The different models just have different shapes (note the 50 is shoulderless and 70 1/2 is not)- it's not a length thing. There is also a model 415 and it's not a sword.

I have a 70 1/2 with an old yellow box with green lettering, and it's my favorite razor. I think it's pretty old. The ones with the checkerboard on the blade and red boxes are much more recent, and I would guess 70s but don't know.

There's also usually carbon steel (not stainless) equivalent model numbers that have the same shape as their friodur cousin.