Look on the transmissiom casting for a number. I think there is one there, cast into the case. Other than that, go by the model--horse, pony, etc.
As for parts, MTD (I think) has bought out the Troy-bilt name and has access to parts for older machines.
Check back thru the messages in this forum. There is a lot of info in them!

If the serial number isn't on the right side of the tranny (flat cast surface sticking out beyond the top cover), I believe the older models had the S/N on the top of the belt/pulley housing.

The manuals and parts lists can be found here:

http://manuals.mtdproducts.com/mtd/public/list.jsp?BrandCode=10001

Type in 'horse' for the model number and then your serial number.

This is the most comprehensive parts supplier I've found:
http://www.custompartsinc.com/cat.asp#ret

I have recently purchased a troy bilt horse rototiller serial #221660. Does anyone know the manufacture date? Thanks

Early 1976

My troy built tiller won't free wheel so I can move it around, without starting the engine. Does anyone know what the problem could be?

i recently purchased an older model 8 hp horse, the only number to identify it is on the right flat area stamped in- #657975 any ideal what year this is,,,,,thank you,,,,,lou

Made February 1982