Oh, that’s a bummer, @thenewreligion. I’m sorry if I helped lead you wrong. I do still think you got a deal on those tools, assuming you find a use for eyelets those sizes.
It sounds like that gun-holster company is using typical US grommet sizes, like you’d see for grommets in Tandy stores, but numbered in their own way: #6, #8, and so on. I believe those are also the sizes Osborne tools are built for. I’d personally prefers those sizes for small goods and bags, rather than boots and shoes.
That said, I’ve seen a bunch of hobbyists use those grommets, since they’re so easy to get in the US. I used 3/16s on my first pair, for example.
I’ve also seen some sign of pro makers using them. DW mentioned using #00s, which are the 3/16" size, in Western Packers. I’ve also gathered that the PNW makers keep that hardware and setters around for those who request wider top eyelets on tall logging boots for easy tying.
I ended up moving away from that sizing system to the old USMC sizes mainly because the USMC sizes just look right to my eye. Looking at a bunch of shoes and boots, I guess I’ve just become accustomed to the sizes and proportions of A and AA especially. Overall diameter is one thing, but the diameter of the flanges compared to the diameters of the eyelets also turned out to be a legible visual cue for me.
Secondarily, I also find it hard to source US sized grommets with the right depth under flange. Grommet for fabric and sheet work, like tarps, were too short for heavier or lined upper leathers that I tried. Some vendors sell taller ones, but they’re way taller.