Great photos and annotations!

One additional tip too for why we should bother pulling the rubber slowly is to also avoid stretching too which will actually change the length of the rubber and have it no longer sit flush on the adhesive like it once did.

Another gotcha too is to note the (+) and (-) terminals on the battery as originally installed. The bad thing about this battery design is that it is symmetric in space which makes it possible to insert it the wrong way! Very dangerous. Take a silver marker and really amplify the terminal pins and note how they are meant to align into the battery slot. Don't get it wrong or you'll short things.

Also worth mentioning since you got the unit open to blow it with compressed air and do any cleaning you can take care of with certain parts easily accessible, eg get that fan cleaned out, etc.

happy fixing folks, good luck!

Turns out not to have been the field assembly but the anchor screw which somehow fractured previously and was shortened by the previous owner. I replaced and clamped down and set all the screws I could down with loctite made, adjusted the power screw and I was back in business.

I dropped a pair of Styliner ii trimmers from countertop height and the motor stopped vibrating effectively, it only drew the armature to the field assembly, never repelling it to create the oscillations. It still vibrated but it was more quivering and chattering if I pulled the armature away. Something broke in the frequency or I'm not sure what exactly. I'll be ordering a replacement field assembly and see how things go and report back here. Not sure what could have failed in the unit that would cause the field to set one polarity.