It's going to be 47,48 or 49 with the lever action shocks as they went to tube shocks in 50.
Scroll down the page and it shows the year to year differences.
http://www.cs.siena.edu/~lederman/tr...ignTrucks.html
The 47/48 would have had the gas tank mounted to the inside of the right frame rail and you could probably see where the mounts had been. They also had the center mount cab mount in the middle of the crossmember that ran under the back of the cab.
48 would have had the cab mounts on the side of the frame. That second mount for the cab in the third photo throws me though as I have never seen that on any AD frame I have seen. The burb/panel guys might throw some light on that
The front crossmember appears to be Chevy and it only having one center crossmember means it should be a short bed frame.