SBI= Studebaker International
So do you not have a tailgate; or is the one you have beat to crap?
As Lee told you there are two versions of beds for the C cab trucks. The earlier ones are narrower than the later ones.
1956 is the first year for the wider beds; but it would be a good idea to measure the inside width just to make sure. If you are looking at a tailgate it is pretty easy to tell the difference by the number of ribs above and below the lettering. If it has three ribs it is for a narrow bed, if it has two ribs it is a wider bed.
As far as the new tailgates from Mack Hills the 56-59 ones are much stiffer than the 49-55 ones, and I'd have no problems using one.
Keep your chin up, parts are out there you just have to look and ask. Trust me it's much easier to find stuff now that we have the internet.