We're sad you're here. No one is sitting on a page looking at buying a $.92 barrel spacer unless something has gone wrong in their lives. We're sorry for your loss. Your former barrel spacer will probably come out from under the fridge one day covered in dust. You'll get it back. Or it will get sucked up by the vacuum, and you'll never see it again. *Sigh* Better to have loved and lost, I guess.
Anyway, the Lander Barrel Spacer is part number P0060. It has a flat on one side that fits into the liner of the Lander. It's threaded for a T6 frame screw. You may need a torx tool with Wiha bits to fix this one, but let's be honest: if you're buying a replacement, chances are you've already gone to town with an existing bit driver. Well done.
We recommend adding a drop of Blue Threadlocker to the screw threads when reassembling your thumb studs. This will prevent the studs from sneakily escaping into the abyss.