My guess is that someone wanted an additional stanchion, sometimes to add a gate. The original deck mold had raised places in he tooling to let the stanchions stand up proud of the deck surface and this would allow an angle, or eliminate an angle to the attachment to the deck. Duplicating the factory raised base required an angled piece of material, and someone chose teak. Teak would have been a common choice at the time.
Nowadays we would use G10 or one of he denser plastic products. Mill it to shape. Or... make a perfect wedge with a 3D printer.
Since it's long past time to re--bed your stanchions anyway, best just remove them and re-bed them all. Prolly also good to add some G10 backing plates underneath while you are at it. I just hope that the coring has not suffered from water intrusion.
Edit: the sloped spacers I added under our stanchions are pictured in this earlier thread.
https://ericsonyachts.org/ie/threads/stanchion-bracing.3371/#post-18646
