Looks like an inductance plate... old trick to beef up the sound.
Seeing the pic, I wonder if someone didn't add the plate then put back the original sticker... But it might be a SSL2 with different specs for some special run, as suggested by the plain cover. Who knows? Not me, so I'll shut my big mouth.
I also thought about an OEM product. The ground connection looks very factory like made and the sticker is on the baseplate.
BTW its on sale at Kleinanzeigen. I asked the seller for permission to post the pic.