04072157
BELT REPLACEMENT ON HUSTLER FASTrack
on the mower deck
It is the model number
Well, this link would be one, it is under Excel:
You could check with the Old Lawn Mower Club
Write down the serial number which should be printed on a serial number tag somewhere on the mower. Take the number to your local John Deere dealer and they should be able to tell you what year your mower was produced.
I believe it a engine on the back with valves
606
That would depend on what sort/make/model of mower it was and what country it was made (and possibly sold) in.
I have been told it is the first two numbers in your serial number.
Usually on the bottom of the seat.