it gotta be an H. they didn't use "M" ....I saw a pic of it and it has the older style recoil on it. does it have a side exhaust? you need the ser. # after the code numbers to tell the year. should look something like this as an example, H25 25145G SER 2243R hard to tell the year without any pics though!
it's a 2-1/2 horsepower Tecumseh. "sidepooper" they call them with the side exhaust. most likely used on a tiller or something? the serial number starts with a zero. could be from 1970 or 1980. i'm leaning towards 70, just by the condition it's in. there is another way to tell, by the way the recoil is held in place. either rivets or screws but I cant remember which is which, rivets first or screws? if you get rid of that belt drive, and put a clutch on there, you could use it for a mini bike!
