www.pendemonium.com has this to say:
QUOTE
Esterbrook model 444. These have a red gasket set inside the bottom glass portion of the inkwell that sometimes goes missing over the years.
The proper replacement gasket is as far away as your local hardware or plumbing supply store! Look for the "Flush Valve Seal" for a Mansfield #210. They're even red like the originals!
The last batch we bought came from our local Kempker's Hardware Store, a True Value outlet, and this is the info from the package:
Danco Flush Valve Seal for Mansfield #210
Two other numbers on the package: 340 and 88096
The UPC code is 3715588096
Specs of the Esterbrook Model 444 gasket:
Outside diameter is 79 mm
Inside diameter is 55mm
Thickness is 1.6mm
It's a flexible red rubber
William