The fact that the packing unit came out of your pen is an anomaly. Sheaffer did not intend them to come out, but sometimes they come loose, most often on these early war time Triumph pens. Sheaffer's instructions said to drill out the packing unit and replace it. We don't have that luxury.
Richard Binder and I spent a year playing with these things before we started to take them in for repair, and concluded that the safest way to restore the pen was to take the nib out and drill out the packing unit from the inside. The blind cap must come off to get the plunger rod with its head gasket out prior to drilling out, and rebuilding the packing seals. By the way, we rarely find these pens working at all, let alone working properly when we find them.
The back end of the pen must be absolutely air tight and water tight. If it is not air tight, the pen will not fill much if at all. Pushing the plunger down creates a vacuum in the barrel, which is released when the head gasket reaches the part of the barrel where the barrel widens. A bad seal means a bad or no vacuum, which means that the pen won't fill, and is likely to flood.
The packing unit was alternating layers of grease impregnated felt and rubber. Eighty years later, the grease has evaporated, and likely worn, and the rubber hardened. The Viton 0-rings we use are both chemical and wear resistant, and with a little silicone grease from time to time so that the plunger moves freely, should last a very long time. I have some that were restored 11 or 12 years ago, and are still going strong.
The head gaskets found in these pens were graphite impregnated rubber. They need to be flexible so that they maintain contact with the barrel wall, and can flex forward and back depending on the direction that the plunger rod is going. They're usually hard and rigid now, or missing all together having hardened and fallen apart. They must be replaced. The rubber should be a high grade buna-N rubber as close to 26 thousandths thick as you can get it. The stuff we use is 27 thousandths. Most rubber on the market is too soft, and either 15 thousandths (way too thin) or 31 thousandths thick. The little washer that you see is a backing washer, which is there to limit how much the head gasket flexes, providing the rigidity needed to create the vacuum as you push the plunger down.
I've tried solvent welding the packing unit back in the pen, and using shellac. Neither has proven to be adequate. Given that Sheaffer did not intend these to come out, I use a high grade (and expensive) epoxy to secure the packing unit in the barrel. Careful cleanup of the excess is recommended.
If you are going to the Ohio show in November, I will be doing one more seminar on restoring the Sheaffer plunger filler pens. Not just talking about it, but actually restoring one. I've done it at the two other shows where I did seminars this year, and it's worth your time if you're interested in these pens.