In 1949, SP began converting flat cars with bulkheads for
drywall service. From the outset many were marked "when empty return to
plaster city," a drywall plant on the SP in the Imperial Valley.
I don't have specific construction memories, but I do recall
seeing such loads in the bulkhead cars during the early 1950s.
Before that time, of course, box cars could have been used.

Tony Thompson