I was told it was from an I/O card, presumably a Commodore A2232 serial port card for the Amiga computer
CSG 65CE02-2 (C) (M) (C) CBM 1987, 88 1690 52
HONG KONG HH125907 MP150SG
Sometime between etch16 and etch25 I believe a beaker contaminted with nitric was used. This caused unintentional staining of the chip as well as etching in some of the less pure areas of the chip causing pitting.