I am currently rebuilding and parting out a total of 7 Pro levers from 3 different sellers and came across these two different pistons as well. My best guess is that there were two models of Pro levers.
The old ones have the same piston and piston seals as the Sport lever (right in the picture), a pivot screw whose head stands a bit out and - most important for you - an adjustment screw whose ball at the end is slightliy larger than the new model's. They will also need a brass washer with a slightly bigger hole which go along with the old model.
The new ones have their own specific piston (left in the picture) and seals, a pivot screw with a flat head and vice versa an adjustment screw with a slightly smaller ball with their specific brass washer. The black body also features a small white logo on the backside, which the old version does not have.
I can post pics in the coming days if there is any interest.
So yeah, unfortunately you will run into issues, most serious one is that your current brass washer won't hold the adjustment screw in place, it will slip through. Also I think the ball head won't fit. Only if you exchange all the other parts, except the pivot screw of course, everything will be fine. These guys in Switzerland sell a full set:
https://www.velo-direct.ch/Produkte/Artikeldetail/FE-058.301/
From my observations, the old style might actually be the superior design as non of the brass washers of the 3 I have were bent by the constant pressure against them while with the new style levers all of the 4 I have had bent upwards a bit which results in lever play. Might be due to the old ones being ever so slightly thicker and the pressure is put on a larger radius.
Btw i get the rounded head out of the lever with a ring plier.
