Hi Wayne,
30 years ago sheaves used to come in several different profiles, depending on the line to be used and the then-current industry thinking on what was best for life of that particular technology line.
On our prior boat, an '81, I found at least one double-groove alum. sheave, said to be for wire/rope halyards.
The others since then have been more of a "standard" circular section sheave profile, and the spar ones have been alum. sheaves. I have seen a "V" section with a rounded bottom to the V, and rarely a sort of flattened oval.
These days I see only the standard circular profile groove.
Like other owners, if making a change at the masthead, I would advocate going to a ball bearing alloy sheave for the mainsail halyard. This will help with hoisting a load over that 180 degree turn at the top. Otherwise just be sure it is an alum. alloy turning on an "oilite" bearing.
Loren