I've rebuilt AC boxes in 69 Charger and 70 Road Runner. The drain pans were always metal, and the numerous donors I took apart in boneyards were the same. In addition, I managed to dig up a NOS drain pan many years ago and it was identical to the OE piece. I agree with your hypothesis, the 'rebuilt a few years ago' replaced the evaporator with an aftermarket including a matched, plastic drain pan.