[Mr70]

I've seen that nomenclature before on a 70-71 Camaros,and believe it is specifying the color of the actual light bulb itself...i.e. AMBER vs. WHITE/CLEAR when the Rally Sport package was ordered.I think this RPO is telling the assembler to use WHITE/CLEAR behind the white/clear reverse lens.
GM typically used an AMBER colored bulb whenever a seperate white/clear lens was used,per the Dept. of Transportation to offset and standout between the two colors.It's not a well known RPO and was sporatically listed on a few Camaros' docs. back then.Why it's not more commonly listed I don't know,but it did exist and has something to do with the electrical bulb circut on these cars.
Chevrolet resurfaced that RPO recently on the 2002 Camaro-Firebird & C4 Corvettes.They list it as RPO T93 Smoked Lamps,Tail,Stop & Turn,Special.

1970-72 Malibu's had AMBER colored front bumper/fender turn signal lenses with WHITE/CLEAR bulbs behind them.
1970-72 Chevelle Super Sports had WHITE/CLEAR colored front bumper/fender turn signal lenses with AMBER bulbs behind them.
I think notating RPO T93 on your car was telling the assembler to go against the usual norm during assembly.