04072157
2004
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.
606
I believe it a engine on the back with valves
That would depend on what sort/make/model of mower it was and what country it was made (and possibly sold) in.
Usually on the bottom of the seat.
I have been told it is the first two numbers in your serial number.