Pre-owned Rolex mystery

[jssnow1]

Hi. Years ago my sister purchased an old, pre-owned Rolex at a flea market in NY. It appears to be an Explorer whose dial was re-touched in a less-than expert manner; the luminous numbers are a bit messy. In any event, she recently had the watch serviced by a friend who is authorized to service Rolexes. He told her that he is not sure which Rolex she really owns, as the serial number is 51xxxx, dating to 1947 production, and the explorer was not produced until the early 1950's. In addition, he told her there was a "Rolex reference number" of O 81792. So, I assume, somewhere along the road the original dial was switched out to an Explorer dial, but not sure about the rest of the watch. Through reference books, I have since confirmed the serial # and corresponding production date, but I have not come across anything close to such a "reference number". Any ideas? Thank you