Dear Bob: Sorry about the reference to World War II - it should have been World War I. The Kronprinze Wilhelm (by now USS Von Steuben) was scrapped in 1924. I share your caution about the problem of nailing down firsts. I am currently in the last stages of compiling annotated list of topical firsts ...

If the information is still of use and you have not already found it, the first hospital ship on a stamp is apparently US #Q6 which pictures the Kronprinz Wilhelm (as it was at the time Q6 was issued in 1913), and became the USS Von Steuben when the US took the ship in 1917 and used it as a troopshi...