Roinik I believe all P1800 have the remote shifter.
So did 72+ 140s. But the M40/1 boxes had a long metallic cover with a plastic cover underneath, and there's more than one version of that shifter extension. From experience on 140s, '74 M40 shifter extensions are quite different from '73 models.
M45/6/7 had a frame-like affair bolted to the rear of the box, and these were different depending if they were 4 or 6 pot, Diesel or petrol, 240 or 700/900.
@jrhendo do you have the part number of the gearbox in question? It's on a little plate on the underside of the box, usually near the output flange on an M45.