I managed to lose the O-Ring on the back of my TWSBI Eco. It can come off and get lost when being removed from a pen loop. I searched for the size it needs but couldn’t find any info. I contacted TWSBI support and they sent me some replacements.

For anyone who needs the O-Ring for posting at the back of the barrel, the size is:

• 10x1 Metric Size
• OD: 12mm
• ID: 10mm
• CS: 1mm

You can get these at hardware stores, industrial websites, and sites like Amazon. It should be listed as ID × CS, so 10x1 means 10 mm inner diameter, 1 mm thick. Or TWSBI support sent them for a nominal fee.