The 8yp4 test CRT is a very versatile tool that I have used many times, and should work, provided it's hooked up correctly.
It seems you will have the same problem I did, IF, it's not a complete 8yp4 test jig with yoke, which is the thin neck of the 8yp4 vs the thicker 21AMP4A , for that I simply cut up an old TP roll to size, put it around the thinner neck and built it up with painters tape to where the bigger yoke would fit more snug.
Obviously you will have to make a adapter to match the pins between the 2 if you do not have one, but once hooked up right, there is no reason why it should not work, just don't expect a pretty picture, as the deflection angle will be a tad off!
