How long were part numbers re-used?

Have had a Griswold slant logo 791X Regular Kettle for a while now, picked up another, This time a 791B, but this one is an ERIE with patent dates etc. (See Photos)

Would they start with A, then if they were at say, "K" when they changed the logo, just move to new logo and the first ones would be 'New Logo' (Part No.) "L"?

Is there some sort of method to this madness? I'm sure Griswold isn't the only one, and I think I've seen this with other foundries as well.

Griswold called them pattern numbers; Wagner, catalog numbers. The letters after the pattern numbers serve only to distinguish one working pattern from another with the same markings. Revisions to trademarks would often be applied to existing patterns, leaving the pattern identifiers unchanged. There is evidence to suggest that the same letter was sometimes used on more than one working pattern, with another mark such as a small numeral or letter elsewhere on the piece serving to identify the unique pattern. Only rarely did Griswold use a letter in a pattern marking to denote a revision, as in the 151 and 151N waffle iron patterns.

More about numbers and letters on patterns: http://www.castironcollector.com/numbers.php

Thanks for the information Doug.