I'm a Gravely guy and the T1's were built on Gravely L series tractors.  So the T1 is the ultimate Gravely "attachment" for the collectors and a good machine if you need to get into a tight space.  I owned one of the original 3 spool T1s and now I have the 7 spool in the picture I posted.  It's built on a Gravely 500 sreies tractor.  I'm guessing it's about 1972, about the time you went to Power King 4 wheelers.  I can't find a serial number anywhere on it.  Any idea of how many T1s were built?