Yes, I have photos of a prototype trailer. Email me at email@example.com
and I will email photos to you.
Dropping the idea had nothing to do with the lengthening of the M422 to M422A1 wheelbase. The trailers were deemed too expensive to produce in quantity. Instead, the Marine Corps opted to produce a Mighty-Mite compatible version of the M416 trailer, designated M416B1.
The M416B1 had mounts for a pair of water cans, lifting loops at the four corners, and hubs with spacers which allowed for mounting of the deeply offset Mighty Mite wheels. I can also provide you with photos of the M416B1 if you would like some.