You are using your back to assist in traversing the gun to the left or right.( by sliding your back around the ring)
I have seen it the TM ( now I'm gonna have to dig it out and make sure )
This mount helps you keep both hands on the trigger
![Shocked :shock:](./images/smilies/icon_eek.gif)
as oppossed to using one of them ( hands ) to push the trolley around.
I believe you are correct in it being "late war'
Wouldn't the spare ammo trays mount to the leg brackets anyway ?
The 30/50 combo cradle would be the same as the "regular " more commonly seen mounts and would also mount a tray or "ready " box...... as I'm sure you know
Is this the info you are looking for ??????
Hope this helps
John