Yours sounds like the mast heel that was on the mast that came with my 1970’s Mk2, sort of rounded off with a pair of sheaves in a small block screwed into the flared bottom of the mast track.
When I removed the old mast plug, I was surprised to find a lump of lead inside the mast (I don’t think it was an anode) as well as a lot of powdered aluminium oxide. Unfortunately the mast had aluminium worm up near the hole for the pivot pin and at the spinnaker pole ring so I did not mess around with the plug but decided to buy a new mast. Had this not been so I would have taken a hack-saw to the old mast and cut off enough length to be able to fit a new 5 sheave plug (as fitted to the new mast I bought from P&B/Selden). The new plug has a tenon underneath that fits into the new mast track I obtained from P&B, which makes a nice positive support for the mast, securing it from side to side and front to back (against the bolt at the back).
I take down/put up my mast on my own, what is especially good about this set-up is that once the mast plug and track are engaged, the mast is secure and I can jump out to fix the forestay without worrying it is going to fall backwards.