Up for sale is new old stock of rare Casio memory module RC-4.  It comes with new battery.

The memory module is compatible with the following Casio calculators:

FX-720P, FX-720PF, FX-750P, FX-750PF, FX-820P, PB-410, PB-410F, PB-500, PB-500F, PB-500(s).

These are new and were never used before.