Bill,
In case you're still wondering... I've found some more information about the two earliest RCA flys that share the same frame/core. RCA numbers 211T1, 211T3 look like the standard FLY 1 we're most familiar with for sets with 10BP4, 12LP4. RCA part #211T5 appears to use the same core and frame but was intended for early sets using a 16AP4. I believe the dual rectifier version to be the equivalent to a Thordarson Fly 4. Has anyone a Fly 4 in their possession that can verify? They would appear to be a very rare item. I believe the below picture is a dual rectifier version which would be for the larger CRT. Bill, does that flyback you were referring to earlier have any identification number on the frame or box?
