There were many generations of Pre-PX panels. The durability and quality of the sound is different between them.
At least a few versions that I can think of:
-Wooden construction with equally spaced horizontal ribs (original A-1)
-Plastic construction with equally spaced horizontal ribs (more durable and stable)
-Plastic construction with the horizontal ribs broken into different length segments, significantly reducing the “drumming” affect, among other things. This was the latest up to the PX panels. You will see the ribs at the top much closer together than the ones on the bottom with these.
-PX panels. Total overhaul of membrane material, stator wire insulation and construction design and materials. A big deal imo. I believe they did get more efficiency out of these, even with an apples-apples dispersion version comparison.
-PX Panels with Bass Focus. Basically the same as PX, but changed the horizontal rib spacings once again to improve bass response.