I haven't seen any tapered tubes from them as yet. Trumark RR-T tapered tubes are the standard I measure everything against. Properly adjusted, RR-T's are great fun to shoot with over the top styles of slingshots. I have never heard of Robert Blair, so I'm gonna have to study some more.
Ya know, I think the reason I like RR-Ts is because I really s-t-r-e-t-c-h them by pre-cutting them shorter right from the start to get max speed out of them. Sometimes they come from the factory with weak spots by the pouch in the rubber and break quickly, but I retie both ends to the pouch and keep going. Some look really thin when stretched by the pouch and others look thicker. I just go ahead and re-tie to the pouch before shooting on the thinner ones because they will soon develop a break.
Oh Yeah, Are you planning to have a "T" at the top of each fork on your someday 1/2" Metal frame for rubber?