I have a Mopar 3690430 distributor and it has a very quick and small mechanical advance that doesn't seem to be in line with the instructions I've found for it. I have my initial set at 10 degrees at 750 rpm.
Mechanical advance seems to start around 1000 rpm.
By 1100 I have 12 degrees
16 degrees at 1200
18 degrees at 1400
22 degrees at 1600

And then that's it. 12 degrees of mechanical advance. I tried twisting the rotor and it seems to have one light-ish spring and one HEAVY spring. I took my idle up as far as my underhood dwell tach box would go (2000 rpm) and then a bit further and it just sat on 22 degrees without a hint of creeping. I dunno where the rest of this advance is coming in but it's somewhere where I don't like my head under the hood.
Even the instruction PDF for this distributor that I've found in a couple of places suggests mechanical should be all-in by 2000-2500. I took a look at the position of the pins in the slots when I hit the beginning of the heavy spring and it looks like it has around half as much advance left.

Were these things ever set up with this kind of a curve? I've peeked through the openings in the top plate and it doesn't look like it's been messed with, but the rotor is stuck fast to the shaft so that's about all I can tell right now.

Some of the Mopar dizzys are different than others. There is a number stamped on top of the plate that has the slots to limit advance. Whatever that number is double it and that is the total mechanical advance.

The heavy spring you're describing is a governor spring, they are usually very heavy and one side is slotted. They will limit the last 2-4 degrees of advance to 4000 or so rpm. Usually you toss the governor spring when you do a performance recurve.