Am I correct in assuming that there are no markings of any sort on the reverse side?
Assuming such, there are indeed few clues as to whether this happened in Missouri or in California or somewhere in between, nor the amount of time delay in its recovery. If I were investing time into this piece, I would be looking in the Carthage, MO newspapers first. The lack of a second postmark makes me wonder (my best guess on a scenario...) if this was lifted from the sender's mailbox by someone looking for Christmas money enclosures, recovered by postal authorities and then the machine postmark was added at the same time as the official seal the rubber stamp message. As opposed to being stolen in transit or at the point of delivery, which typically would have postmark from a city down the route.