In looking at the Cumbres train registers for the 1936-37 period, I noticed that conductor Hurley was assigned caboose 0503. I would suspect that the caboose on the movie train in its move to Santa Fe and its return to Alamosa was the 0503.
It is too bad the movie train cars were not listed on the dispatchers trainsheet. We know from photos that two of the three cars were RPO 61 and coach 311. Unless there in a note regarding coach 300 being left at Embudo by another train, it is possible that coach 300 was the third car and was left at Embudo by the movie train on its return to Alamosa to reduce tonnage for the little tenwheeler, which had been out of service for some time prior to having been fixed up the movie role.