in
I did eventually converge on the answer.
There is RS274D which uses a separate aperture file, and RS274X which embeds the apertures in each layer file.
It looks like some PCB firms in China changed ("upgraded"???) the software they use, and it can no longer read RS274D. The operators are too dumb to look at the actual filenames, and the default position is "computer says no".
Fortunately my 1995 PCB program has a checkbox for "embedded apertures" and that does the job.
Funny it took 20 years for this issue to pop up :)