It's always good to see a 990 being saved.
As Doug stated, the 3pt. arms are definitely homemade and the muffler is original but turned wrong. It's fairly rare to find an original muffler in decent shape.
The 3 bolt front wheels and early style deck lift arms make it '75 or back. The hole in the frame we're talking about is on the left side at the rear of the front section where it flares. The hole is oval and maybe 3"x2"?? It cracks on the bottom of the hole where the frame is small. It's definitely on '74 and back tractors but I think it's missing on '75s (and newer) so if you have no hole, yours should be a '75.
It has the starter/gen bump out sheet metal. I think that puts it in the early years.
There's three configurations for the bump. The earliest has it high on the grill, right below the headlight area. Then it moved down on the grill to the location you see on this one. I don't know what year it changed but my '74 has it low and the '71 brochure I have shows it high. When they switched from the starter-generator to the starter and alternator ('80ish??) they got rid of the bump completely.
Edited by Canawler, July 13, 2014 - 11:25 AM.