Tailgate interchangeability

I have a '55 R-112 and have been looking for a tailgate for a while.

I found a '59 tailgate that looks like it will fit but all I have are some pictures to go by - I don't even know the model of the '59

any body have any insight on how interchangeable tailgates are among the mid and late '50 pickups ??

I think you should contact, The Binder Boneyard for the information you want.
https://thebinderboneyard.com/

IH tailgate interchangeability, a cautionary tale... Curved sides vs. Straight sides

My 1956 S120 is missing the original tailgate, so when my brother spotted an IH tailgate at a local salvage yard he picked it up. The yard said they thought it was from a 'late 50s IH pickup', but didn't know what year exactly. At first it appeared to be the correct style and size, however fitment was an issue. When we installed it, we found the overall length to be too short as well as the tabs for the chain latch pins to be the wrong length. Upon closer inspection, our new salvage yard tailgate has curved features on the sides while photos of original S120 tailgates have straight sides.

I've attached photos of each style for reference. I don't know enough to clarify every possible size or style of IH tailgate, but I thought I would share my experience. Happy hunting!

1950-1956 L. R. S. all interchange

as above 1957-mid 1960s do not fit,

besides the curved side the easiest way to tell is 1950-1956 tailgate has a round tube top rail,

later one has a roundish squared top rail.