Those guys can be like your 10mm socket. If you drop 'em, they will roll and roll and be gone forever! ;-)
 
Seriously, we lost one in the shop like that! It went on its solo adventure, never to appear again. We have some available individually for a great price if that ever happens to you.
 
For whatever reason, those pucks (yeah any round thing is a puck in Canada hey!) don't stay in place when you remove the tank. They're a loose fit on the stem and fall off more often than not ! 
 
To avoid this, check our article on gluing them for peace of mind: Keep Your Rubber Puck in Place
 
This is a Suzuki Genuine Parts:
44511-12D00-000 CUSHION,FUEL TANK FR