ROM BIOS of the Plus Hardcard 20 Version 5.91 (1987)
This card a the single 64k BIOS chip. Chip marked "B60" See Picture.

Original EPROM M5M2764P 

Socket location U215


Image source: TOP853 Programmer

File Name: HS20U215.BIN

File Size: 8,192


There is also a 1986 version of this card with version 3.31 BIOS