Hi Arlen,
Thanks for the suggestions. The error was on my side; Mark's suggestion of dragging the put.js tiddler wasn't applied correctly the last time, but I was able to see this clearly by reviewing both the terminal output and dev tools based on your note.
So thanks to you both for your help!
Just fyi, but here is how I did my troubleshooting (your feedback would be welcome here, as this would be great for future reference, i.e. if anyone else has a similar issue):-
1. Checked terminal output (for both old vs new html files. The "old" html referring to the file where I was unable to save, while "new" being the empty I downloaded for testing)
[2018-08-06T06:45:09.371-0400] GET 192.168.0.16 304 192.168.0.16 /windows_everybody/_archive/old.html 1.773 ms - -
[2018-08-06T06:45:26.498-0400] GET 192.168.0.16 304 192.168.0.16 /windows_everybody/_archive/old.html 2.448 ms - -
[2018-08-06T06:46:12.639-0400] GET 192.168.0.16 304 192.168.0.16 /windows_everybody/_archive/old.html 2.109 ms - -
[2018-08-06T06:47:32.986-0400] GET 192.168.0.16 200 192.168.0.16 /windows_everybody/new.html 4.243 ms - 2584815
[2018-08-06T06:47:37.873-0400] OPTIONS 192.168.0.16 200 192.168.0.16 /windows_everybody/new.html 1.112 ms - -
[2018-08-06T06:47:37.873-0400] HEAD 192.168.0.16 304 192.168.0.16 /windows_everybody/new.html 1.015 ms - -
So no matter what I did with old.html (creating new tiddlers, as an example), I didn't see any OPTIONS or HEAD methods.
2. Then in dev tools, "old" had just the 1 Total Request, while "new" reflected the requests based on my actions.

3. Since "old" didn't show any additional Total Requests (or PUT in terminal output), I used FF's debugger to view the core modules and noticed the missing "put.js". After properly applying Mark's suggestion (and saving the file when prompted via the dialogue box), I was then able to see the file.

Ultimately, this was my user error is not applying put.js directly, but your additional suggestion of using the browser's debugging tools helped identify my mistake.
Thanks again, guys.
Cheers,
JP