OK Guys now I have the definitive answer on all red versus red/amber lamp assemblies.
Many of you think that this research is "over the top" but that's how we get answers. Call it intense curiouity, but I've owned 4 cars from the 1968-1969 era (Austin America, Austin Cooper S, MG Midget and of course the 113) and all of these vehicles seemed to change from all red to red/amber at around the same time--mid 1968, though at this point in time nobody gives a hoot about all red tail lamps on a BMC product!
I contacted the DOT and NHTSA and it turns out that this is just mere coincidence and manufacturer's choice, not any US regulations for the change. Red or Red/Amber is acceptable and has been for years, for the tail lights. Perhaps there were some European rule changes or it could be simply that both BMC and MB decided to change at the same time for aesthetic reasons.
So, as Bob Geco (krwxbz) astutely noted, the new '05 Mustang is all red. So are many others. The use of red/amber actually adds to the complexity of wiring (per a Visteon engineer) and is done solely for aesthetic reasons.
Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored