Rusty, Since your panel is a 1952, the lock cylinder on the right rear barn door should be in the same type of "fixed" door handle (push button style for 1952 and later VS turn down style for 1951 and earlier trucks) as your main entry doors. The handle and lock cylinder I believe gets assembled together, installed to the door and then the latch mechanism is installed after that (from what I can tell).
Below are snapshots form the 1952 Shop Manual Supplement that detail the process for replacing the lock cylinder in the fixed position door handle. From what I can read, the retaining ring for the lock cylinder is installed to the lock itself and don't see any retaining ring for the latch operating shaft going into the lock mechanism itself.
If I'm off-base with this info, maybe you could add some pics of your handle and latch pieces to help understand further questions??? I'm interested to hear how this turns out.
EDIT: Is the spring for the push button handle the one pictured in Diagram 3 on the first pictured page below???
