Some month ago I found a very usefull sorage box for all my HP-41/71/75 modules. Is is white transparent and can be put into a file. It can hold 32 modules a "page" and is available in Germany by Conrad Electronic:
Holder for 32 modules: 419150 (Klarsicht-Sordiment mit 32-Fächern)
Holder for 10 larger items: 419184 (Klarsicht-Sordiment mit 10-Fächern)
I hope I can help many collectors by this posting.. I had many asks for these numbers...