David,
You are right, they are hard to find, near on impossible in NOS condition.
Something to note.. The Monnins and some very early 844s had a different bezel to the 'regular' 844s and quartz versions.
You'll see that yours probably does not have an inner steel ring between the insert and the glass, but the later versions did.
You have a couple of aftermarket options..
A Rolex Sub insert will fit, but you'll need to sand the inner edge of it.
The other option is... There is a French ebayer that sells parts; cases, dials etc to build your own sterile diver watch. He sells cases similar to the 844. They are different, with a larger lug width, but they come with a bezel + insert that look like a dead match to the Monnins. I did buy one to check it out, but haven't got around to measuring it yet. Cost was aroun 80 euros I think. Yes they are aftermarket, but a good option to restore a watch.

I'll post a link to the ebayer tomorrow.

Stewart
- Heuerville