Water Pump Housing Shims - When are they needed?

[infernosg]

It's been established S4 water pump housings use shims where the gasket isn't present and S5 housings don't. I've confirmed this in the parts fiches and I also noted that the S4 shims have two different part numbers depending the the VIN. What I don't know is what is different between the S4 and S5 that necessitates this. Is it the front irons or the water pump housings that are different?

I ask because I have a S4 front iron and a S5 water pump assembly and I'm trying to figure out if the shims are needed or not. If someone knows, great! If not, I'll try to take some measurements tonight and report back for future reference.