This idea has merrit, and that's why I'm backing it up. No one can say it was a field mod. It was designed from the start to do this using mounts that were already in use. EVERY single thing I've read about the M20 talks about it being mounted on a jeep or other light vehicle using a standard .30 mount. Just because there are no pictures of it doesn't mean it wasn't done.
"A jeep or weapons carrier can easily
carry the M20 recoilless rife using the standard M1917A1 .30 caliber machine gun
tripod mount." (emphasis mine).
I see it saying it was meant to be used on a standard .30cal MG
tripod mount, not vehicle mount. Small difference, but an important one. It was easily
carried by a jeep or weapons carrier, not
mounted on one. Again, small but very important difference. Everything points to it being a weapon designed to be portaged into the field and set up on the ground as any other standard infantry support weapon.
The 75mm versions were shipped to both ETO and PTO according to what was posted earlier (I had always read only PTO service), but the more significant action was on Okinawa in the Pacific after the European War was already over. Since there were not significant Japanese armor formations, the proposed scenario of mounting one hastily on a jeep to meet the oncoming enemy armor column is far fetched to put it mildly.
So, we are back to burden of proof. All the photo evidence presented and the snippets I read in my previous look into this topic all suggest mounting the M18 on a jeep occurred after the war was over. We can suppose it happened all we want . . . without proof, it is still just supposition.
Still looking for the smoking gun - er - RCL.