According to SIP, later model PXs had the metal edge seals. Also according to SIP, the rubber edged ones are 62mm in diameter, while the metal edge ones are 62.1 mm in diameter.
To me, this explains a lot. When a shop replaced the seal on my 2005 PX, they put in a rubber edged one that popped out in a matter of weeks. So did 2 more before I insisted on a switch to a metal one, which failed for other reasons but didn't pop out. In fact, for a variety of reasons 2 more metal ones since haven't popped out either. The shop could not remember which factory seal they originally pulled out, but I'd lay odds it was one of the metal ones.
This also explains why some say to not use a metal one because it'll stretch the seat and a rubber one will never stay in again. So far, so good, the 2 types are 2 slightly different sizes and that's important... don't jam a big one in where a small one will suffice. But what I don't understand is what SIP is saying in the following paragraph outlined in bold..
What groove are they talking about, not the groove for the giant circlip? That's behind the seat of the seal. I don't know because I've never seen the inside of an older PX case.
The oil seals made of metal, from later PX models, can be retro-fitted to the GTR/TS/Sprint/V/Rally models motors, replacing the original rubber item. The deciding factor is whether the seat of the oil seal has a groove cut into it or not. The PX 200 crankcases produced by MALOSSI do not feature this groove, meaning they can also benefit from the use of this seal type. If this groove is present within the motor the rubber version must be used. Attention to this difference can be crucial for an oil-tight motor!