Although I am not an expert, I've been shopping for Kawai 52" pianos that have sostenuto pedal. Most sellers of these pianos will claim that these US series pianos are "predecessors of the K-8". While they all look like the K-8 (manufactured since 4/06), the functional difference is that most of these US series pianos do NOT have sostenuto.
The following models have sostenuto, and here are their manufacturing date ranges (model start date and model discontinued date):
US-75 (9/85 - 8/99?)
US-8X (9/90 - 3/91)
US-95 (9/85 - 7/89)
K-80 (7/98 - 1/02)
I am not sure at this point if the US-70 (9/82 - 8/85) or the US-80 (9/82 - 8/85) have sostenuto or not.
IMHO, the differences in Kawai model numbers correlate to manufacturing dates (anniversary editions, etc) more often than specific features; as one model gets discontinued, a new model number will be used for a very similar piano. There will be different features that are incorporated into these models as time goes by.
Here are more manufacturing dates for pianos you listed (these do not have sostenuto):
US-50 (9/82 - 8/85)
US-55 (9/85 - 7/89)
US-60 (9/82 - 8/85)
US-65 (9/85 - 7/89)
US-63H (3/88 - 12/89)
There are also the US-?X series that are somewhat newer:
US-5X, C, LE (3/91 - 3/93)
US-6X (7/89 - 3/91)
US-7X, LE, RS, SE (3/91 - 3/93)
US-9X (9/90 - 3/91)
The following website contains dimension and weight data: http://www.tiantianaiyue.net/page/kwy/30.htm
(You can scroll to next/previous to view other piano models) There are subtle styling differences (curved furniture legs, squarish cabinets, etc) between different 52" model pianos.
Hope this helps!