As far as I know, there is no 545SS. From my contact at Perlick, I know that they did an initial run of them that did not meet Perlick's standards. I am unsure if they ever got a successful run produced, but I never heard of one.
Also to follow up on earlier posts: I still use the 545PC's thru the wall of my kegerator with great results. I still cannot get them to work with my kitchen taps, which pour from a box kept cold by a blower, without excessive foam. I think these are a great option for keezers, but I don't think I'd recommend them for anyone pouring from a tower or other setup.