Ok - Pulled the old mast base out. Lots of corrosion and goop. If you want to use all the features of the new base, you need to trim away some of the sail track (about 2"). So far I've gotten everything except for the clam cleats off, have trimmed the sail track, made some epoxy repairs, cleaned off a lot of old crap. The new base fits well - this is going to look/work nice. It does sit lower on the mast ball though.

So - there was no plug. I kind of get the feeling that maybe the base has been off once before. It seems to have just been sealed with run of the mill silicon, but there was some evidence of some other sealer from a distant past (was white, kind of looked like maybe 4200). So I'm on the fence as to what use when I put it in. Regular old silicon? I also have some silicon/poly sealer, I have some 4200. Not sure what to use. Any opinions?