I also wonder if the French, being staunchly metric ever had "metric" drive sockets?
Not so much a metric drive, but Facom did invent a new type of drive called CDX which was
a 3/8" drive size, but could transfer 1/2" drive torque. The trouble is there are a squillion sockets
in the world and apart from a teeny tiny few they are all 1/4", 3/8" and 1/2" (obviously not
including the larger drive sizes) and because they have been around for over 100 years
trying to sell a ratchet that uses a completely new drive system or a metric sized square drive
would quite simply mean you would have to buy a whole new set of sockets to fit it.
It's basic economics for companys, if they want to sell ratchets make them in the sizes that
most people will have sockets to fit them. Needless to say the oddball imperial drive sizes and
the Facom CDX drive didn't make a big impact (pun unintended) in the world.
I once did a thread asking how 1/4", 3/8" and 1/2" became the norm in socketry and
the best answer someone came up with was...
1/4" = 2/8"
3/8" = 3/8"
1/2" = 4/8"
It seems logical when you look at it this way, and because of the amount of sockets in
the world I wouldn't expect to see a change in drive sizes for many generations. Making
any kind of new drive size, metric or otherwise would just not be good business.