Okay, so I'm a bit C64 and c128 biased when it comes to Commodore 8-Bits. (And I've got a soft spot for the Vic-20.) But I haven't had much experience with the PET, Plus/4 or other machines like the C16/116.

When I saw some random chip adapters in the online store at Retro Innovations my first iteration of the Buyer's Guide just left those out. But when I went back and tried to expand the listings I decided to look into these adapters.

I'd never heard of a 6540 before. But I found a few resources online that might help you out. This c64-wiki post, MOS Technology Products, lists all the chips that MOS made. And according to this document, the 6540 was used as the BASIC ROM in the PET and PET2001 computers. It's a 28 PIN DIP that came in a around 25 different revisions, and I'm guessing that none of these is readily available today.

If you've got a PET, and you need to put a newer JEDEC standard 2764-27512 EPROM in there, this adapter is your adapter. I also found this document, which is essentially a spec sheet for the 6540. This is the documentation I've linked to above. It's not technically for this adapter, but this adapter is a drop–in replacement for the 6540.

It looks like MOS 6540 had some Chip Select options to help map it into the right address space. This adapter includes a 74LS138 decoder/demultiplexer that sits underneath the EPROM to implement that feature of the 6540.