I think the change from the Europa TG to the Europa TR was 1965, maybe 1966... Someone probably on sl113.org cleverly found a mapping between the serial numbers (first letter) and corresponding year of production, but I seem to not be able to find that...
Yes, "someone" ("BMB") on the German W113 site researched that
but only for radios installed in cars delivered in Germany. His verdict is the following (translated):
Until September 1964 only tube radios of type Europa TG, Mexico and Grand Prix TG were delivered.
After October 1964 the transistor radios Europa TR, Mexico TR and the top of the line Grand Prix became available.
Thus for Sead's 1964 car with a 516 code a Grand Prix TG could be correct depending on the delivery date.
Unfortunately what the factory installed in cars exported to the USA under code 515 is not known. It is very likely that it was a Europa MU (AM/FM).
We should not forget that the primary partner for everything electrical in Mercedes cars was Bosch. That is the reason why Studebaker/MB dealers preferentially installed the Bosch Blaupunkt radios in 1964; MERCEDES-BENZ of North America (MBNA) was registered on March 15, 1965.
In another post "Charles 230SL" (
https://www.sl113.org/forums/index.php?topic=32090.0) wrote that his DataCard shows option code 515 (Radio Becker, AM, FM, USA) for his early 1964 230SL. Afaik, that would be the earliest date that code was used.