JMP50 = Plexi 50 surely.
+1
One could mod the Plexi to a JMP in minutes. Nik's amp's are straight, period-correct clones, just look at a schematic and make the changes. It could seem silly to make him invest in fabrication just to get a different faceplate.
Plus, the Marshall Plexi RI is exactly the same as a '74 and later JMP (it started off was a JMP RI and they just switched the faceplate and renamed it), down to the PCB layout. Of course, you can also just buy a real JMP for less than even Nik can sell one.