NOTE: sacs between #16 and #18 are the most widely used. #16 tapered is special-purpose, for Eversharp Skylines and Parker squeeze-style converters. #14 and #15 are the usual sizes for smaller and slenderer pens, while large pens such as Parker Duofold Seniors will take sacs from #21 and up.Sheaffer PFM and thick-barreled Touchdown pens (first-year and Tuckaways) use #17½ necked sacs; #15 is the size for slender Touchdowns, while Snorkels take #14.
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