Hi Dave, Thanks for your pictures that really helps and with the links i got from Graham93 between you both that sorts out were the confusion was coming from.

Cheers Jon.

Hi Jon
This is fairly typical for the period and type of vessel.
Basically two bands round the mast support a small shaft that can rotate in bearings in each band. The shaft has a flat spot on each side in the middle with a hole. The mast has a slotted shaft attached to the boom that also has a hole through each slot. This fits over the shaft attached to the mast and a pin or bolt is used to attach all together.
Some vessels had a rotating boom to reef the main sail but I can't see this detail on your drawing.
The system will work well so you should have no problem in it's use.

The gaff is a boom with two side cheeks attached and shaped. There is a support on the mast to hold the gaff and Parallel beads are attached across the open end to hold to the mast

https://images.app.goo.gl/FX87cvWuEVsaH4Mw9

Not that difficult to make and will look correct on Katie.

Some pics of my Cariad rigging to give you a bit more detail. I used brass but wood is more typical.

Have fun

Dave