I just found out they these two were real production models!
The 28 is the just like the 31 but with the old GE engines. (How useless! You only have 4000lbs of fuel! You better be looking down the runway after 2 hours and not need to go around. Although...FL510 in 14 minutes! Wow! )
The 29 was just like the 28 except with more fuel...you get 5500lbs! (A whopping 3 hours!)
Anyway, just thought it was interesting.