Hi Michael
I think the origin of your 300SL is a clue, it's a Euro car is it not. This is almost certainly what is left of a pair of "winter" mats for the rain and mud in Europe. The drivers side probably got worn out through use (the dreaded accelerator heel wear plus you've a manual so that's both feet!) and thrown away and the previous owner just put it underneath the drivers side mat in case they had any passengers with muddy feet come aboard!
Suggest that you could do the same to protect your passenger footwell as I see you've been having "some rain"
in the US recently!!
If not try a sale on eBay - some people will buy anything!
Cheers
Jim