For the two digit models:

If it's in the 'teens it's a K-frame
If it's in the 20's it's an N frame
If it's in the 30's it's a J-frame
If it starts with a 6 it's stainless

But that's not hard and fast because with a two digit numbering system you run out of numbers fast. There's plenty of examples that are outside those ranges, plus there are ones in the range that don't conform.

There's the Model 61 which is the .22 Escort. It's in with a bunch of stainless revolvers. What's up with that?

It's just all over the place.