The SP at one time had a former short RPO that was used as a caboose by a local crew on the Colma branch if I remember correctly. I suspect it was listed in SP's records as a caboose, because it was used as a caboose. It would not surprise me that the ICC might have had some kind of regulation requiring that. A lot of the record keeping in those days was done pursuant to ICC reporting requirements. That might explain how 04892 got designated as a caboose, but presumably it should have been changed when it went on the wreck train in some other service.
JBWX