Ok lets use a C-120 as a example, 12hp, the 0 tells you it's a 74 to 77 tractor, narrow dash tower, gas tank under the hood, those are the easy to see items, C-121 built 78 and 79 wide dash tower, gas tank under the fender, iso mount engine, C-125 built 80 to 84 Black hood all but 84 had plasic fenders, from factory, most have been changed over the years, also a isomount engine, yes there are exceptions to the rule in fact quite a few if you dig.
The older tractors IE, 653 first no is HP second no. 5 elect. start, last no the year, only used in the 60's so 1963
if the middle no was a 0 it would be a pull start, and yes there are exceptions to this rule also like the 701, 702 they are elect start.
Guys they all were back then stick shift, till 1965 then it was a 7 for the middle # for the hydro, and all hydros are elect. start, the middle # tells you if elect. start or not, like a 656 is a 6hp elect. start 1966, the 606 is a 6hp pull start 1966