Stem on this folder is labeled "Cyclo Man". My caliper reads 21.2mm however it tends to not be very precise.

Does anyone know the correct stem diameter for this bike?

Probably 22.0 since it's french.
-Gene-

There is a 21.1 mm quill stem diameter, but I think (don't hold me to it) it was only used by Schwinn and Columbia. The French stem diameter was 22.0 mm, and the standard diameter was 22.2 mm. A difference of a whole millimeter is a lot to miss, so I think it may be a 21.1 mm. Let's let the experts chime in though.

Its actually 21.0mm
Seriuosly

You won't find a quill that is long enough for the rise required in a folder.
My secret....

Use a 20.8 seatpost (old bmx size) and shim the top with 22.2 -> 25.4 shims, this allows for a threadless stem on top.
Although if you just want to replace your cycloman stem I have one with only minor rust marks I can send you.
Personally I wanted drop bars so I had to get a common size stem clamp size.

The bmx post is not going to be on a shelf in a bag but there are probably a few in your town somewhere.
The top 30mm of the post is expanded to 22.4 to accommodate the seat clamp and using the shims I managed to work a 1" threadless stem on. (Quite snugly)

I've never heard of a 21.0 mm stem, would you mind educating me on what sort of bikes it was commonly used on?

you can try this:
https://www.niagaracycle.com/product_...ducts_id=12077

Close but the original bars are not 25.4, ill have to go measure them but they were some archaicly small diameter.