More prodding has found the problem with the clamp...
First, I tried the original 28" express barrel that came with the gun. The extension and clamp all worked as designed.
The lug on the short barrel (the ring that fits over the mag tube) was bent/crooked. (this is an old 870, 2 3/4" chambered, non-ribbed barrel that I cut down to 18 1/8") I had never noticed the bent lug before the extension. Some very careful application of a soft-faced hammer has remedied the mis-alignment. Now the extension fits well, as does the clamp.
And I sure did wrestle with that spring
Thanks for all your help,