I'd vote for the unique side-release original belt mechanisms - one thought - many USA cars have brand-new condition belt buckles as our seatbelts did not come into common use until the late 1990s.
Also, the driver's side retractor would be a like-new passenger belt retractor assembly.
Remember - active and passive safety was the hallmark of this series of S-Class, and any changes might be less than desireable to these systems.
I'm planning to replace Max's driver's belt system with a passenger side system from Aussieland, if anyone knows of a like-new belt (my webbing is worn) I'd like to get the belt/retractor assembly.
There should be no shortage of belt retractors in AU - just make sure you post a picture of your buckle style as there is an alarming amount of different versions.
An interesting note is that SOME USA spec fasteners are unique in that they have the "fastened seatbelt" switch incorperated into the fastener with cable harness. I dont know that the fasten seatbelts warning was legally required anywhere else on earth..
Having said that, I do remember on old volvo's of the same era in australia being fitted with the same system, but they also had a pressure switch incorperated into the seat base to detect if someone was sitting in it. When this failed or become unhooked youd have to fasten the passenger seat belt even when no one was in it, to stop the "fasten seat belts" light flashing at you constantly.