I got sucked in to the Estie thing just today! I bought 2 desk pens they were only $5 each! One appears older perhaps the front section is hard rubber? it has several bug bites and #2556 nib, the other is in better shape, likely newer, has a #9550 nib and I can't tell if the nib section is hard rubber or plastic. Does the direction of the name and numbers make a difference in vintage? The 2556 is across the nib where the 9550 is lengthwise down the nib. They both have had sacs replaced at some point, the 9550's is nicely done and appears to have been a professional job, where the 2556's wasn't "shellaced" in place and appears to be too long for the pen body, kinky folds at the bottom.
I'm not 100% sure about this, but judging from several observations I've made, I have concluded that the numbers going across the nib, like your 2556, are how Esterbrook made them for the majority of their production, and then in their later years (mid 1950's perhaps?) they started doing the lengthwise numbers. I don't know the exact year though, and would love to find out the answer to that. For the longest time I thought this was exclusive to the 9xxx series of nibs but then I ran across a couple 2xxx nibs that were also lengthwise.
I agree, if the sac is scrunched up, it was probably too long or the wrong size. As you get more and more pens, you'll find some really horrible repair jobs. It would be a good pen for you to repair if you're new to pen repairs! (I only own a few desk pens and haven't bothered restoring them, but if I had to guess, I'd say the sac size is 16, but someone else can verify that for you.) While you have the pens apart, I'd be sure to test the other one with water to make sure the seemingly good sac is leak-free.
Did your pens come with pen bases/holders? You can find them on eBay often if you need one or two.