I think you may be out of luck. I don't know of any 1/72 AM doorguns. Neither the above PE guns nor the Dragon M60s in their HMMWV kits are M60D models, they are standard M60s. The D model has no forward stock on the barrel, a ring sight on top, and a dual hand grip in place of the buttstock.
M60D Doorgun model
The M23 Doorgun mount is actually pretty easy to make with some plastic rod or tubing and some sheet styrene. Here is an explodede view to help out.
Here is a site that gives more info on the M23 mount and other UH-1 armament systems.
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