aftermarket dude
the 27c256 bit is the type of chip, ie 28 pin 256k eprom
the VPP 12.5 is the voltage required to blow the eprom ( load info on it)
the standard fare is a 27C64 28 pin UV eprom, not often used now as the 256k are used in Pcs for the bios and are cheap... as chips
HTH
Product Development and Endurance for Delphi.
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