1) The venturi inserts both look like very large bore items. 2) The plastic mount on the one suggests more likely not stunt, tho some do use them for stunt, it's much less likely. They are used for FF, tho. 3) Most the rest of the engines look like they are hotrod engines, without throttles. The one on the left is almost certainly a Webra Speed 10cc, for example, and the bottom center one is an MVVS .40 to .49. Two engines top center do appear to have throttles, however, and a few I really can't tell. The one top center left is an MVVS R/C 3.5 or .25 (is there a .28?) for R/C combat, fairly hot engine. I'm wondering what the engine in the bag between the two Jett .40's is...maybe an HGK .40. Looks brutal, but I think they were not too good. Upper right is a K&B .40 from about '69-'71. They can run real strong, with good torque. One I worked over ran an 11-4 Rev-Up at 18k on 40% nitro. Good FF engine, not so much for racing, but legit for Quicky Rat (I think it's called). Weak crank, and not a long lived engine without some good rework and very regular rebuilds.
My guess...and it is a guess...is that these were used for AMA Free Flight gas models, and the ones with throttles just hadn't gotten setup with a venturi yet. Plus, I don't recall ever seeing or reading about a RO-Jett .40 built on a cast case. I think the RO-Jett .40's are all BSE's. But you could certainly shoot the picture to Dubby or RO by email and ask. I'm guessing that they're not stunt engines, anyway.
Steve