As far as I can tell, the type number is not really decipherable. It refers to the list of options the mower factory has specified, such as whether to have a cage over the muffler, paint colour, primer or choke, etc. My current guess is that if there is no existing Type that matches a mower factory's order, Briggs creates a new number - and it may simply be the next unused number in the Briggs book of Type numbers.
If you ever find a rational system attached to the numbers, please let us know.
