Hi Brad. Although you have a very early 1969 140, unless you have had a serial number research done through Two-Cylinder magazine, I have no idea if it was built before the Nov. 25 cutoff on the scans above. If it was, then it should have GT-3 tires on it, which are 16x6.50-8 front and 23x8.50-12 rear turf tires. As to the different GT codes, that info is in some owners manuals, sales manuals, price sheets and sales bulletins. A breakdown of them is as follows-
GT-1 4.00/4.80-8 2ply front 6-12 turf rear
GT-2 4.00/4.80-8 2ply front 6-12 traction rear
GT-3 16x6.50-8 2 ply front 23x8.50-12 turf rear
GT-4 4.00/4.80-8 4ply front 23x8.50-12 traction rear
GT-5 16x6.50-8 2ply front 23x10.50-12 turf rear
GT-6 16x6.50-8 2ply front 23x8.50-12 traction rear
GT-7 4.00/4.80-8 4ply front 23x10.50-12 turf rear
GT-8 16x6.50-8 2ply front 23x10.50-12 traction rear
For the 400 the following two codes were used
GT-12 18x8.50-8 2ply front 26x12-12 turf rear
GT-14 18x8.50-8 2ply front 26x12-12 traction rear
So far these are the only tire codes I have found, but I haven't spent a ton of time going through my library looking for them either, so I'm not sure if GT-9, GT-10 or GT-11 were ever used (I'll be honest and say I am a bit suprised that there wasn't a 4.00/4.80-8 4ply front 23x10.50-12 traction rear listed where I looked).
Kent