I guess I figured this out from experience over the years, so I shouldn't fault anyone for not knowing, but I get annoyed these days when people say things like, "I don't want to get a 21mm lug width watch because it is so hard to find 21mm straps for it." I even read a post from several years ago from a guy who seems to pass himself off as some kind of expert claiming that Omega deliberately made watches with 19mm lugs so that you would be forced to buy straps from them, "pure greed" he said. Funny that my three Omegas all have either 20mm or 18mm lug widths. I guess they decide not to be greedy from time to time. No, companies design band width based on what they thing is appropriate/proportional to the watch, whether we agree with their decisions or not.
So, let me now inform all those who do not know- YOU CAN PUT A 20mm STRAP ON A 19mm LUG WATCH! No problem, and no special modification or expertise needed. Just put it on like you would any other strap. You can do this with NATO straps, leather straps, and rubber straps. It's not going to work on metal bracelets unless you file them down a bit, which counts as a modification, so I won't mention that here. Take a look at one mm on a ruler. It's tiny. Then divide that in half for each side- tiny!
Here is a Grand Seiko with 19mm width lugs. It came on a bracelet, but I decided to put a NATO strap on it for a change of pace. No special order or special effort here- simply put a 20mm NATO on it.
Here is a Nomos watch with 19mm lugs. Again, I simply put a 20mm leather strap on it. It really wasn't any harder than putting a 20mm strap on a 20mm lug watch. I've got another Nomos Orion with 19mm lugs on a 20mm alligator strap. Looks great.
Now here is a 20mm lug watch with a 22mm strap that I took off another watch. In fairness, the strap is probably closer to 21mm. Maybe 21.5mm. No problem at all putting it on this watch. The biggest issue with putting any strap on this watch (even a 20mm strap) is that he spring bars fit so close to the case that it will no accept any strap (other than the metal bracelet it came on) without using curved spring bars.
Here are a couple of watches, one with 17.5mm lug width and the other with 18mm lug with, with a 20mm NATO strap. As you can see there is some bunching up on the side here which doesn't look so good, but it still went on, and it could be notched to fix it, but we are talking about 2 to 2.5mm bigger NATO strap here. A strap that was only 1mm larger would not be a problem.
So let there be no more talk of not wanting to get a watch because the 19 or 21mm lug width would make it difficult to find optional straps to put on it. Just put on a strap that's the next size up and be done with it. Easy to do and it looks good.