Go to any hardware store and they’ll have a much larger section for American-size bolts and machine screws than metric. Presumably this is because they still sell significantly more of them. Metric fasteners also tend to be more expensive, which I assume is due to lower volume. For non-machine thread fasteners, I’ve never even seen metric ones for sale. So in a new or renovated house, they’re not likely installing the drywall with metric drywall screws. If you have a deck built, they won’t likely be using metric deck screws to put it together.
So, American company, requesting custom machinery built by an American firm, and you’re specifying that the fasteners used be metric?
Why do you hate America?! 
J/K. Good to see you’re with one of the companies embracing the change. As I said I’d rather everything just switch already. I’m sick of needing two sets of tools.
Heads up. 1/2’’ and 9 /16’’ don’t have close metric matches.
7/16 x 25.4 = 11.11
1/2 x 25.4 = 12.7 mm
9/16 x 25.4 = 14.29
5/8 X 25.4 = 15.875
Only the 5/8’’ is close enough for the ‘‘wrong’’ wrench to work. Well maybe the 11mm and 7/16’’, but I see little of the 11 mm. Since 16 is a power of 2, halving or doubling 5/8’’ also fits.
On lumber. I do remember 2 x 4’s shrinking, but maybe in the 50’s or no later than the early 60’s.
3/8’’ is enough smaller than 10mm that you could never mistake the 2.
Really? 9/16" ***does not ***equal 14mm? I’m fairly certain (99%) that either size open end wrench, will fit either size hex head bolt/nut. :dubious:
12 point and 6 point sockets on the other hand, I’m not quite so sure about. 
I will do some comparison testing, and post my findings, ASAP. 
(Just as soon as it gets to be daylight, I’ll go out to the shop and double check.) 
Actually, our products have been 100% metric for years. Heck, we share design responsibility with Europe and Japan. But the companies that build our machinery are German and Italian – although the Italian one is just the current owner; the original company has a long and proud US history.