I've learned more about o-rings than I care to think about, but I think I have the size figured out.
I asked P.A., and one of their techs said 11mm x 3mm. At first I thought he was just rounding off the 111 sized o-ring at 10.77x2.6 but it turns out that there IS an 11x3mm o-ring.
I have several of those o-ring kits, the type Harbor Freight and many others carry, it looks like what they SELL as a metric o-ring kit is simply the SAE kit with the label converted to metric! So the "metric" 111 is just a 7/16" x 3/32" SAE converted to metric dimensions. Hence the odd sizes. In other words, the metric kit is just an SAE kit with a different label.
In Europe, they have actual round number metric o-rings such as 11x3, 9x2 etc.
So the so called metric kits you can buy here are only going to be useful some of the time since they are not actually metric o-rings. Good news is 11x3 is available on Amazon and a dozen other places.
I guess if my double stacked 111s ever fail I can then buy and fit the proper 11x3mm o-ring and be done. So far they are holding up.