It seems that in 2017, the authors of curl realized that this 'expect 100 to continue' was broken on many web servers, and decided to remove the curl behavior on newer versions of curl.
I so happen to have WSL / Debian, so trying with curl 7.81.0 was really.
The curl on my Windows labelled 8.4.0 with a release date of 2023-10-11.
So - I'm not sure which curl version is best, but I had it working on 7.81.0.
Firefox, and Chrome failed me. maybe I should have tried MsEdge but I won't be trying further.
thank you to the Forums for the help!