These photos are of the sheave from Constance, #262, and are provided courtesy of Jonathan Bresler.
Sheet aluminum panels were inserted on each side of the sheave, acting as block cheeks, to keep the main halyard from wearing on the slot in the mast. Instead, the halyard often wore the aluminum panels, and sometimes created enough space for the wire halyard to jump the sheave and slide down between the sheave and panel.
Aluminum sheaves like this one seem to be standard on the newer Alberg 30s.
Jay Davenport provided this photo of the main halyard sheave from the top of the mast on Revolution, #526. It is aluminum, 6.75" in diameter, .5" thick, and has a 7/8" center hole in it. The hole has a brass or bronze bushing in it which reduces the ID to 5/8".
This sheave has carried a wire rope for 25 years with little apparent wear. Jay changed to an all-rope (3/8") halyard, which the original sheave can handle without machining.
Replacement Sheave Design
According to my calculations an aluminum one made like my drawing would weigh 22.4 oz where the delrin would be 12.75oz
Using a Block
I tried hard to use the mast sheave and absolutely agree it would have been better but hanging from a bosun's chair at the top of my mast I simply could not get the 3/8" halyard line to pass over the sheave and I did not have a wire to rope halyard available and I wanted to go sailing, so my quickest option was to shackle a block and use an all rope halyard. That was 5 years ago and I haven't felt a need to replace it. But someday if I ever have a need to lower the mast or if the current system breaks I may try harder to use that sheave."
As Gordon pointed out there are some definite potential failure modes here and I would not recommend this system for hoisting a bosun's chair. This system works for me and has for many years without the use of the mast sheave and without internal halyards but there are probably better and safer ways to attach blocks to the masthead.