I've noticed my Polks do not have the serial # on the back like the picture I've attached. They do have a little yellow strip with 50382 above the inputs.
I purchased mine in 1992. I remember I purchased them from an NYC electronics company via a catalog. The company was pretty reputable if I remember correctly.
So my questions are Did they not have a serial # on them at that time? could I have bought fakes? or am I just paranoid?