Looks like 1988-1989 for the 1720. It says its a Lawn Tractor. But my chart only goes up to 1989. It may go into the ninties, but thats just a quess. Sorry I can't be of any more help.
</font><font color="blue" class="small">( the gas tank is under trhe seat and the mower has a kohler 17 hp engine, seller told me it was 8 years old. Dealer says at least 14 who's right? )</font>
The only ones I found for sale were 1990 models. One was $1495 and one was $110 (that must be a cream puff - no picture).