[HOTFUN]

I have had two brand new GM 64-66 low hump mats in my lifetime and the first was bought by me in high school from the Chevrolet dealer in hometown back in the middle 70's and had the factory part number molded on it. I remember it well as it came in no box and the dealer had it on the shelf folded square and with a GM parts shipping sticker on it. I stored it for years thinking I would restore Dad's 66 someday and later installed it in a restored 66 truck I finally built in the late 90's. Last one I bought was off eBay 4 years ago and it has the factory part number 3865525 molded in the center at the rear of the hump just like Yelmer has pictured. If I didn't have it so well rolled up and stored in the storm shelter I would take a picture. I feel all the original GM parts department mats would have the GM part number molded and as said above and the mold vendor may have made their own line of JC Whitney mats after GM quit ordering their resupply.
Maybe Powerbrake guy could make that part number piece in rubber like his power brake pad section and glue it to the mat if you wanted numbers matching. Another thought is it might be possible to carve out of a ripped up original mat and rubber glued into place too.