All I could find is the official part numbers for encoders SW6 & SW7.
It is :
Part Number _____________ Description
452-08891 _________ SW,RTY,GRAY CODE,16 POS,PVB
Unfortunately there are no specs in the description...
Edit: I had similar problems with the encoder on my LXP-1, it was jumping values.
I sprayed a bit of deoxit in the pot without disassembling it and its performance was improved.
These early lexicon encoders are famous for failing after a few years of operation.

If you need the service manual let me know.