I know this thread is a little old but I'm glad I came across it as I also thought I was going crazy, since I have one of these masts too. I even considered whether it was an alternate part but it is embossed with 4844 on the underside of the base so no mistake about what part it is supposed to be.
In my case it's in a 6274 Caribbean Clipper set that I originally received as a child and recently got back out of storage - however, it's possible it was originally from a BSB as my brother had that set and the parts might have been mixed up. I'm 99% certain both sets were regular retail purchases in the UK and not from any unusual source.