Has someone a module of this?

[Dave Frederickson]

All I can find is a listing in the 1985 T&M Catalog.

HP-41, HP-71, and HP-75 Custom Keyboard Overlays
HP-41: HP 82501A
HP-71: HP 82487A
HP-75: HP 82721A
Relabel the HP-41, HP-71, or HP-75 keyboard with special userdefined functions assigned to each key. Available in a variety of background and printing colors.

... and this in the 1986 catalog.

HP-41, HP-71, and HP-75 Custom ROM Modules
HP-41: HP 82508A/B, HP 82509A/B
Provide 4K or 8K bytes of memory with each module, or nearly 21,000 program lines with up to four 8K byte modules.
HP-71: HP 82491A/B, HP 82492A/B, HP 82493A/B, HP 82494A/B
Provide 16K, 32K, 48K, or 64K bytes of program storage in a plugin module. May be used in quantities of one to four for a maximum
capacity of 256K ROM. (Minimum order: 100 modules.)
HP-75: HP 82726A/B, HP 82727 A/B, HP 82728A/B,
HP 82729A/B
Provide 8K, 16K, 24K, or 32K bytes of program storage in a plugin module. May be used in quantities of one to three for a maximum
capacity of 96K ROM. (Minimum order: 100 modules.)

So, there were at least 100 modules out there somewhere.

You might try giving the folks over at https://groups.io/g/hp75 a ring.