clinthohp20mm is the approx lug width, outside to outside. It is NOT an integral strap, it can be removed, but is only about 10mm inside, so you might need a special band to fit it. I'll go back and ask my jeweler about it.
CaptpYes, it is a 20mm width but to answer the original question you cannot put a NATO strap on this. Well, you can but why would you use a 10mm NATO on this. Also, the clearance between the lug and case may be too small to wrap even the thinnest NATO strap available. It's integral in a sense that it wasn't intended to be replaced. I guess I should've said non-standard. Yes, it can be removed but, I don't believe anyone makes straps that will accomodate this watch.
See the photo I posted on my Seiko 5 somewhere here in the discussion where I cut a leather strap to mount on these types of Seikos. If you want to replace the strap, you pretty much have to retrofit one yourself like I did.