The Power Cartridge by K.C.S. was a wonderful addon for the C64 (and C128). Those extra functions and speedloader was amazing back in the days, like you had a new machine.
Besides my original KCS and manuals I have several revisions in my collection.
Information:
http://ar.c64.org/wiki/Power_Cartridge
https://www.elektronica-assemblage.nl/geschiedenis/en
https://www.richardlagendijk.nl/cip/cartridge/item/power_cartridge/en