The listing I have shows different weights between 40480A and 40698. Not entirely clear, but it looks like the weights have different effective masses, so swapping weights would change the advance curve.
But usually, the springs determine the curve, and the lower arm determines maximum advance by how far the arm sticks out. The document at
https://www.triumphowners.com/uploaded/34/111457_86lucas_dissy_tuning.pdf shows a selection of arms for comparison. Watch out though, I have seen arms that have been modified to allow
more advance, without changing (or crossing out) the stamped number.[/
Thanks for the info Randall. Seems there is a lot more to it than I thought. I'm just going to keep one of the TR3 distributors as a spare for the 3 and I'll use the existing dizzy in my 4A after I check it over to see if everything is functioning properly. Good to know that because of different advance curves you shouldn't swap one in that's not appropriate for the engine.
I did send my 3 dizzy to Advanced Distributor years ago. He requested the mods to the engine for the rebuild. I told him about the header and street cam I had on the 3. I guess he takes all that into consideration in terms of springs and advance curves because after I installed the dizzy the 3 has been running great ever since.