All US built cars upto 73 (what I know of) came with amber lenses.
That is definitely not correct.
Prior to 1963, all cars in America came with colourless clear front turn signal lenses and colourless clear bulbs. The front position ("parking") light and front turn signal light functions were done with white light.
In 1963, by voluntary industry consensus (no regulations existed yet), amber was adopted as the one and only colour for the front turn signal function. Because most cars in America had combination park/turn lamps, this also meant amber was adopted as optional to white for the front position light function.
So starting in 1963, vehicles sold in America came with either amber lenses and colourless bulbs, or colourless clear lenses and amber bulbs. Both setups were used as a matter of styling preference at the option of the automaker.
If we're talking about post-'63 A-bodies in the American market:
All 1963-'65 A-bodies (except the '64-'65 Barracuda), the '68 Valiant, and all '76 A-bodies came with amber lenses.
The '64-'65 Barracuda, the '66-'67 and '69-'75 Valiant and Duster, the '66-'74 Dart, and the '66-'69 Barracuda came with clear lenses.
All cars built for export was equipped with clear lenses.
Not necessarily, no. At various points in the '60s and '70s, other countries started adopting amber front turn signals instead of the previous white. However, no European-approved amber bulb was available and most countries outside North America did not (and do not) permit any colour other than white for the front parking/position light function. So cars sent to those countries that required a white front position lamp and an amber front turn signal tended to get amber front turn signal lenses and separate white parking lamps (often integral to the headlamps).
It's more than a little difficult to speak categorically "All ______ got ______" when talking about export-spec cars of that era; regulations differed widely by country. Some countries accepted US-spec vehicles, some accepted US-spec vehicles only as private, one-at-a-time, noncommercial imports, some required full or partial compliance with European or individual-country regulations.
