"Looks like you are missing the ‘collar’ for your bracelet pin which is why it’s falling out. Haven’t had a chance to find where you’d get a replacement but I’ll have a look when I get a chance. I’m guessing you can get a new pin and collar as part of a set..."
20mm according to this:
http://forums.watchuseek.com/f21/help-me-find-my-lost-clasp-pin-seiko-frankenmonster-seiko-5-skz255k1-494776.html
The "watch guy" here gave me some info:
"Looks like you are missing the ‘collar’ for your bracelet pin which is why it’s falling out. Haven’t had a chance to find where you’d get a replacement but I’ll have a look when I get a chance. I’m guessing you can get a new pin and collar as part of a set..."
http://www.pmwf.com/Watches/WatchTools/BraceletSizingTools/AFBraceletSizingToolUse.htm#HOW%20TO%20resize%20the%20SEIKO%20SKX779K/SKX781K%20MONSTER%20bracelet
http://www.pmwf.com/Watches/WatchTools/BraceletSizingTools/AFBraceletSizingToolUse.htm#Where_to_find_these_collar_spares_on_the_net