Hi All. I have a Sheaffer Jr. lever fill that I recently won at e-Bay. I would like to replace the sac (I have done several sac replacements, mostly on Esterbrooks) but the more I try to find out information about the pen and the sac to use in the pen, the more confused I get. I have looked on sites like The Pen Sac Co. and others, as well as a blog, but since I don't know the exact model or nib size, I am in the dark. I simply don't understand the numbers and letters they use for identification. Can anyone here help? Thanks.