338-6503 Rockwell, a R65C02 with an obscure number

I saw an advert on bey (China seller) for a Rockwell 6503 IC. The 6503 is a variant of the 6502 with limited number of pins to 28, the same 6502 core with less addressing space.
But something in the advert made me wonder, a 40 pin 6503?

I ordered one and indeed a 40 pin IC appeared. So this is not a 6503!
Then I searched the internet for the 338-6503 printed on the IC and found references to it being a R65C02.
And indeed, the 6502 testers recognized it as working 65C02. Weird!