These Shaw designed Pearson boats of this vintage have some 'quirks'.
When the mast compression post was installed, they drove a wooden wedge under it to tighten the bulkhead that supports the mast. This wedge was then covered with fiberglass. The problem is that the glassed in wedge is in the bottom of bilge and over time becomes wet and rots; and then the bulkhead that the compression post is joined to becomes loose, etc. The compression post doesnt support the mast, its function is to stiffen the bulkhead, the bulkhead is what supports the weight of the mast.
All this is easily reparable, even if the stub bottom end of the companion post is rotted.
The symptoms of this rotted wedge is that the bulkhead will now be moved from its original place ... the door way into the head will not close or the door will jam. What to look for initially is that all the hardware on that doorway should line up. If the bulkhead has shifted the door lock will not work - all this indicates that the compression post and or that wedge under it is rotted and has to be repaired/replaced. If there is servere rot in the post/wedge then fully expect that the mast base will be 'dimpling' into the top of the coach roof (another possibly expensive repair).
Almost all the Shaw boats have waterlogged deck core under the stanchion bases. Look for spider cracks emanating from the stancheon bases into the decking ... if so then most probably there will be extensive 'wet deck core' - a PITA time consuming or expensive repair.
All these 70s Pearson built Shaw designed are VERY 'sweet' sailing boats, very robust, other than the characteristic problems outlined above.
These boats have 'floppy' hulls, and need to be very carefully blocked when on the hard or (temporary) hull deflection can occur ---- never ever put a block under the very aft end of the keel of such boats; always put a vee poppit jackstand between the rudder and the prop and one at the bow.
hope this helps