That's frustrating (and strange
).
If you look at the ASV search link from their website above, it shows the 550 as being for the S4R and S4RS. Did the levers change during the model life I wonder?
Might be worth sending an email direct to ASV for clarification.
(Yours is 2006 isn't it?)
https://asvinventions.com/pfresults/...S4RS&year=2006