To help with a bit of clarity
NATO (technically G10) straps have a split section (a Y) that you thread the main body through, so that, should one of your spring bars fail, you don't lose the watch.
ZULU straps (much less common, and not to be confused with Zuludiver, which is a brand) look similar but are one piece and lack the split.
And then there are standard canvas straps, which look similar but are two-piece.
Aside from the array of colours (including the old and new 'Bond' NATOs), you can also get them in various materials, such as leather, if you want to try something different.
They add a bit of thickness to the watch, but if you don't mind that they are very comfortable (plus cheap and durable). Got mine off eBay, all for under a fiver each.
They're easy to change too, so you can mix up your look easily. I have white, baby blue, sand and Bond versions.
Nice couple of field watches you have there.