I have a specialized with Rock Shox
I have a specialized with Rock Shoxs << I go with this one

Maybe it comes from adding the word "shocks" and transfering the plural to the brand:

I have a specialized with Manitou shocks
I have a specialized with Manitous

I guess it is the same as "clip" and "clipless". Pleople got used to say "clipless" to the shoes and pedals that actually CLIP together.