Mike,
There was an updated top lever return spring AND plunger. When Perazzi came out with the "new" MX-3, that is when they changed the plunger design to the one spring and plunger. Whereas prior to that, the plunger was similar to the TM-1s, and using two springs.
That first style of plunger and spring had a spring with a small diameter. The new springs have a diameter of .172" (4.37mm). The plunger also has a larger diameter (partial)hole to accommodate this spring. The new spring will not fit into the old plunger.
I'm not sure, but when the MX-3 Special came out, is when Perazzi went to the larger spring.
Now, as a side note...There are a few MX-8 style of guns) that use even a larger spring with it's own plunger. This spring measures a diameter of .214" (5.45mm). I've only run across two ever here in my shop.
So, to what MIA was stating, yes..updating the spring and plunger will extend the life of the spring.
As for the gun popping open, we've been through this before...the spring should only close the top lever...tight. The angles on locking bolt and the angles on the bbl are what should hold the gun tight. The locking bolt should not move, even without the springs.
Doug