Happen to have one here... in a box along with a CDBA...
Mask is marked "Avon", so along with rubber boats, rubber suits, and rubber duckies, add FFM's to their list of fine quality British rubber goods.
Doubt that they are in production any longer. Agreed that they are fairly comfortable. They turn up as surplus now and then. If I had to buy one, I'd call DIVEX up for starters and see if they had one spare.
You might be interested in this:
CDBA Rebreather Teardown
Modestly believe that it's a bit more complete than the previously cited article, and pre-dates it considerably.
The CDBA is remarkably good kit, considering it's design vintage.
Dave