Well I got this sort of cleared up in my head. A JMP can be a 2 hole master volume like the 2203 and 2204, (as in JCM's) AND it can also came in a 4 hole non-master vol. 1959 and 1987 (as in plexis) << that's what confused me - I thought those model numbers were for plexi heads only .
and then there is the plexi story.
The Marshall Shopper's Guide - Part I: Marshall's "Plexi" Era