I have been running an RT-1439 in my M996A1 ambulance for years now and recently decided to update to a PRC-138. I didn't want to outright replace the 1439 and and didn't want to add another mount because of space considerations and reasoned I could come up with a way to mount the 138 into the Sincgars mount. It's a fairly simple solution actually. I sectioned about 1 3/8" out of one of the 138 wide plastic battery boxes and installed the power connector from an old AM-2060 harness I had laying around. This shortened the length of the RT to allow some new hold down clamps to hold either radio into the mount. Now I can slip either RT into the truck depending on what I want to do.

Sincgars:
Sincgars radio setup.jpg

PRC-138:
20191002_150318.jpg