no they don't make U-models anymore, the last year for the U-model was 1990. my Dad had 4 U-models (3 single axles and a heavy tandem) he was in the paving business and like the u-model b/c of the short nose and really tight turning radius. the cabs are extremely small in them though, thats why I got a R-model, the cab is more comfortable and they have a longer wheelbase, the u-models would shake you to death! but most of my Dad's work was local in town/city so the ride didn't really make a difference, whereas I am on the highway a lot and like the smoother ride.
my R-model has AC but it's not on the roof, that may be aftermarket AC on that U, or maybe the U-models had the AC on the roof b/c there wasn't enough room under the short hood.