Parker co-developed the lever with another company, Hyster stuck with the other company. Parker did a better job with the details.
http://www.parker.com/literature/IQAN/Literature%20files/IQAN-LST_uk_datasheet.pdf
If you are already disassembling the levers to try and clean/lubricate them, you might try leaving one or two of the lever mounting screws just snug so that any distortion of the housing doesn't distort the lever base. (Doesn't always work but worth a try)
ISZ
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