Well I happen to have the original Kelly Blue Book sheet from 3/1/1971, which lists all the Mercedes Benz models with options (USA versions). Under radio, it shows:
1) Becker "California" AM, Fact. Install. (I believe this was standard on some models, including the 280SL; optional on 220,220d, 250 250S; n/a on 280SE coupe/conv. & 300SEL)
2) Becker "Europa" AM/FM $172 Retail - All (except 280SE Coupe/Conv. & 300SEL)
3) Becker "Europa" AM/FM Stereo, $289 Retail - All (standard on 280SE Coupe/Conv. & 300SEL)
Also, an Automatic Antenna was $88 Retail - All (standard on 280SE Coupe/Conv. & 300SEL).
It should be noted there is no mention of a tape player anywhere. I have seen Becker cassette players that were separate (usually installed in the glove compartment). When they were offered is a mystery. The 1972 350SL had a cassette player (I believe it was a Becker "Mexico" or "Grand Prix") as my neighbor had one since new.
Best Regards,
J. P. Mose
1968 250SL
1987 560SL