Ahoy Norm & Ian,
The original boom extrusion was actually round. But we will ignore that for now. The next standard boom was what is now the Le Fiell L-106. (not the L-100 or L-108) It did not originally come with the sheave weldment . When you order from them now, you have the option of having the sheave weldment. The next size bigger boom extrusion is the L-104. It is also available with the sheave box weldment. All things considered, IMO, the larger, L-104 is the better choice.
The price can go way up with the addition of tangs, bails, cleats, winches, exit holes, and goose neck.
Is this the right answer to the question?