Interesting product from "Holder for M" from Vintage Revival Productions. Here it is, described by Patrick Ng at his blog:
" It is an ingenious two piece product made from a combination of leather and denim label material. One piece acts as a holder which allows you to insert your M into your jean's back pocket (8,800 yen), the other is a pen holder (4,500 yen). Both pieces are looped so that you can slide to the front and back cover of you M easily without permanent damage…"
Those are awfully expensive.