I am not sure about GMCs, but on Chevrolets the clamshell doors were the only rear doors available on Suburbans through 1949.
No one has yet come up with GM production numbers for specific models of GMC trucks during all the New Design years (late-1947 through mid-1955).
Chevrolet trucks during this same period were called Advance-Design series trucks. There were about 15,000 Chevrolet Suburbans made in 1950 (less than 4% of the total number of Chevrolet trucks made that year). I have only seen production numbers for 1950 and 1953 Chevrolet trucks. 1950 was the highest total truck production year during this time period.
If you are planning on trying to restore to original specs, you should join
oldGMCtrucks.com discussion board and also peruse the information at Rob English's
oldGMCtrucks.com website. He has paint color information amongst his extensive documentation on pre-1960 GMC trucks.
Here is the GMC Maintenance Manual that you will find at most reproduction parts vendors.