Extremely useful gadget
For Monsieur Adaptateur
Simple, yet very well executed idea. I got myself two of those right off the bat. I used to try to get my cable-converting needs covered by making and/or getting various cables of as many connector combinations as possible. But sooner or later Murphy gets you anyway and you find yourself in a "how to get those two connected" situation you wouldn't have thought of before.
It there's anything I miss in Monsieur Adaptateur, that'd be screwed-on 4 mm banana posts capable of connecting fork spade connectors. The existing ones are nice and compact but won't let you use fork spades. And I like to use them sometimes for my PSU connections as I found them to provide more robust and stable connection for high(er) currents than using most popular types of banana jacks. But hey, you get two sizes of male blade connectors on Monsieur and I have an insulated connector crimper always at hand (and you should have one, too!) so no worries here. This, and perhaps a female RCA socket wouldn't hurt. But then again, it'd make the product bigger and BNC-to-RCA adapters are readily available.
Long story short: great thing, get at least one - it'll come in handy sooner than you think!