Very nice watch with aviator look affirmed. Personally, I prefer leather strap watches. For my part, I took a brown Hirsch that goes very well with its slightly vintage side.
Wes.Mason
If you go with a true 21mm it should fit the lugs perfectly--worse case you might have to shave the edges a bit with with a razor blade.
The tricky part about these watches has to do with the position of the lug holes, they're drilled VERY close to the case, meaning its tough to slip any kind of strap between the OEM spring bars and the case. If you run into that issue, the solution is to replace the stock spring bars with Curved Spring Bars like these:
https://drop.com/buy/seiko-sna-flight-alarm-chronograph-watch/photos#photogallery
Good luck!
Very nice watch with aviator look affirmed. Personally, I prefer leather strap watches. For my part, I took a brown Hirsch that goes very well with its slightly vintage side.