Thank you both for your responses.

It seems that my convoluted first post created some confusion:

The fork is no longer the original fork; whether the new fork is french or not, I do not know, but everything fits on it (including the headset pieces from my older 22.2 mm stem/24tpi fork). If at all possible I want to avoid changing the fork. The fork, though it accepts the 22.2 mm headset pieces, takes a 21.1 stem. This I know from using calipers and also from trial; I cannot replace the stem with a 22.2 as that would mean also replacing the fork! I specifically bought a 21.1 stem to fit this new-used fork (the original stem was cut with a hack saw; I'm not sure what it's diameter was).

The only thing is that the headset cap piece does not sit tight against the 21.1 stem as it was designed for a 24tpi fork and 22.2 stem. Nevertheless it screws on tightly to my mystery fork. I'm wondering how much of a role the headset cap plays in keeping the stem tight and in place. Is a makeshift fix possible (i.e. could I use hockey tape on the inside of the top cap to make up the one mm dif, so that it is flush against the stem)?