The original pin that slides in the track is just 3/8'' round stock that it rounded off on the ends so it slides smoothly in the track...{ I like to JB Weld them in when I replace them }.

There is not much documented in any of the parts books when it comes to the 1947-1950 underbed carriers...Here is what I have been able to locate....
Attachments
Spare tire carrier5.jpg (92.93 KB, 51 downloads)
Spare tire carrier7.jpg (36.9 KB, 48 downloads)
IMG_3677[1].JPG (135 KB, 48 downloads)
IMG_3675[1].JPG (73.98 KB, 44 downloads)