Advance springs

Here's a show and tell post
https://www.forabodiesonly.com/mopar/threads/360-tune-up.435792/post-1972523041
and maybe this helps too.
https://www.forabodiesonly.com/mopa...-accurate-with-no-load.451803/post-1972806078
There's other guys here that also are very familiar with this stuff. Both Newbomb Turk and Halifaxhops have distributor machines which lets them do some off car stuff.

The heavy spring slows down the mechinical advance before 2000 rpm or so. This avoids having too much advance when the engine gets heat soaked. Drag racing is different. But drag racing the heavy spring is still and advantage because it can offset the retard at high rpm.