[longbow]

Neat-o. The one I had in mind goes way, way back... to 2001, manufactured by Hutech Astronomical Products. Perhaps not surprisingly given the passage of time, their basic website is not helpful in locating this plate device — all I have is a printed sheet with the title Pentax Battery Eliminator Use — see phone pic (scanner being serviced)

Seen that: Hutech Astronomical Products - Pentax Battery Eliminator

That's how I figured out what was installed on my 6x7, thought it was missing the button. Advance Camera had a parts 6x7, so had them replace the plate while he checked out the camera. He noticed the original had threads in it, so must have particular purposed. We both did research and found it was a factory mod used for astrophotography, which might also explain why the camera was in such great shape. He said the camera's condition inside were well maintained and shutter speeds dead on. I still have the plate, if I ever try astrophotography it's easy to put put back in..