NEW OLD STOCK short necked latex sacs, size 18 (diameter of 7.15mm or 9/32" inch in the widest portion). It's necked for a tighter fit, especially on smaller section nipples. Size 18 is one of the most useful sac sizes, and will fit a wide range of popular standard-sized lever-filling and button-filling pens. Overall length is 2 1/8" (54mm) so may be on the short side for pens with longer barrels. However, it can easily hold slightly over 1.0ml of ink. More than most modern fountain pen's converter capacity.
I have attached examples of it fitting to a regular sized vintage Parker Duofold button filler and a waterman level filler. If you trim the necked portion, it can even fit large pen like the vintage Parker Senior Duofold (the Big Red) without any problems.
NOTE: Latex Sacs opening is very flexible to fit into different size of nipples. Please make sure that your barrel will be wide enough to accommodate this sac. Typically barrel with internal opening wider than 8.0mm will accommodate without any issues.
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