Wow - many thanks, Ian!
On my SN the support post is just a single length of pipe with no holes or bolts at the bottom. It just goes straight into the pyramid grp base.
I think our SN's are similar vintage, so it's possible a former owner of your boat may have cut the original post and added the two-piece "sleeve" over it and the two bottom bolts. Hard to tell from the picture, but it looks like there's a smaller diameter post showing at the bottom, which may be the original Amel one-piece post.
I haven't yet tackled tightening this from the bottom, but the sleeve would be a much easier solution to making the height adjustable, rather than replacing the entire post, as I initially had been thinking. Appreciate your pics.
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Craig Briggs - s/v Sangaris / SN68  Tropic Isle Harbor, FL