Rolex did make gold-capped watches up to around late 70s / early 80s I believe, for example some reference 1500 Oyster Perpetuals.
If that is really a makers stamp on your watch, it should be 14k or 18k gold, although it's quite am odd place to put it. But there are very early references from the 40s & 50s which were quite quirky and don't follow the guidelines as we see in newer references.
Getting more curious. Maybe you should take some clearer pictures and send to Rolex SA to get their advice ? Or to a reputable publication like Hodinkee etc ?