My second most favorite topic, gunsights!
Paul, if your range dial came from a P-51 crash, it's going to be from a K-14 gunsight, definitely not a Mk.18! Shoot me an e-mail if you want to see one, I'll send ya a pic of a K-14 & Mk.18 side by side.
The range units as pictured above w/ John's rig were not used on the K-14. Both the K-14 & Mk. 18 had a range
dial, which was located on the left side of the gunsight. Only the Mk.18 had the range
unit, which I'm really not sure why it was even there in the first place.
Basically, the "ranging" for both these gunsights was accomplished by means of a cable attached to the range dial on the side of the gunsight. On the K-14, there was a twist grip attached to the throttle, twisting it in one direction would increase the "range" (or size of the reticle), and twisting it the other way would of course decrease the range. On the Mk.18, this was done by foot pedals.
So I really don't see why a range unit was needed at all, but it obviously served some function, as it was fed into the junction box. I have some pictures of a Mk.18 installed in the Martin upper, and it clearly shows the pulleys & such necessary to route the range cable to the gunsight. Plus, I have a drawing of the complete rig, and it too shows the range cable routed to the gunsight (and the range unit, as the cable is looped around it as well.) Perhaps it was just a redundant component?