Another trick, if you have straps hanging around...many will be stamped with the width, like a 20 or 18. Hold up one or two that look close...it's easy to tell at that point. The right size with slip into the gap easily and fill it completely. Too big has to be slid in; too small will always have a bit of a gap.
And then, of course, you can always google the model number of any recently produced (say, 7-10 years or less) model. Jomashop often has the lug or band width; not on this one. Amazon does show it: 20mm. And then of course, there's the manufacturer's site, BUT this may be limited to only models in the current catalog. Yours is a few years old, and it actually isn't listed any more. If there's a way to dig thru the B&M site to find the numbers on older models, I'm not motivated to find it.
The manufacturer's data is the most reliable, when they publish...not everyone does. (I personally consider that a significant, altho not inherently fatal, deficiency.) But, I have seen errors in the numbers from both Amazon and Jomashop.
BTW, I'm not saying other sites won't have it. The 2 mentioned are *big*...they carry a boatload of makers and models online. So they serve as some of my primary investigative tools.
