It’s hard to tell what something is just by looking. When we work on old Jeeps, we usually look, feel, touch, smell and even taste(!) to figure out what “stuff” is. We can’t do that just from a picture, so we’ve got to make a best guess.

It looks kind of fuzzy and fibrous. The field coils on those old starters were wrapped in the 1949 version of friction tape for insulation. If the armature had, for some reason, dragged on the field coils, it could have worn some of the tape insulation off the field coil. But, I am going to think that you looked the fields over and didn’t see any worn insulation or signs of abrasion.

One thing we have discovered about those early push-in starters is that the lever needs to engage the bendix gear into the flywheel before the switch energizes the motor. If the motor spins up before the gear is engaged in the flywheel, you can grind some gear teeth - not a good thing. The button on the switch can be turned to adjust engagement if needed. On my ’48 truck, the switch was pretty well arced and burned up and I ended up replacing it even after the starter guy overhauled the starter. BMorgil ground some flywheel teeth until he discovered just what was happening with his CJ 3A.

But, the fuzzy stuff you found - it could have been a field mouse nest or some other critter that moved stuff into the housing. If it keeps working and working well – claim credit for a fix and move on. If it happens again – go back and start over.