Whisperflow motor rebuild -- extra gasket?

I've successfully replaced the bearings on my Whisperflo pump, and now I'm ready to put the bells back on the ends. It's actually an aftermarket replacement motor, not the original Pentair motor, but everything looks and acts pretty much identical based on YouTube repair videos I watched.

I found out that the little wavy washer goes in the rear bell, but I have an extra rubber washer/gasket/whosiewhatsit I'm looking at and I can't for the life of me remember where it came from:

WhisperFloExtraPart.JPG

Where does this little dude go?

Side question: how do I know when the shaft seal (the two-part one in the seal plate and impeller) is bad? I have this thing pulled apart and it's about a year old, so I thought of getting a Go Kit and replacing all of the seals, but I honestly don't have a clue how often I'm supposed to replace them. If it matters, there was scale build-up on the bolts connecting the seal plate to the motor front bell, and scale in the bowl of the bell as well. It's a saltwater pool and it runs pretty much 24/7.
 
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