I was looking through a book I had picked up a year or so ago called
"The Elementary Commodore 64". Towards the front amongst the
description of various types of peripherals available for the C64 under
the title "Other Gadgets" was this:
Z-80 CP/M -- This cartridge goes right into the cartridge slot to turn
your machine into a Z-80 base computer enabling you to access the vast
array of CP/M software. With over 2000 CP/M software programs
available, there is little you will not be able to access.
Dan Rector