The original size was 185HR-14. The section height was about 78 (sometimes rounded off to 80) in today's terminology. The W113 was originally delivered with Firestone Phoenix, but by the time of the 280SL, many cars were fitted at the factory with Continentals, again in the 185HR-14 size. You car could have had either.
The 205/70 gives the same rolling circumference, but is 20mm wider. Surprisingly, since this size was widely used at the time, the 205/70HR-14 is almost extinct. Lesser speed ratings can be found, though.
The Coker tire is not made using the same molds as the original Firestone Phoenix. The tread pattern is entirely different. The Coker tire does have rub "humps" on the sidewall and comes in a whitewall, which many feel is important.
Vince Canepa
1967 250SL
113.043-10-001543
568H Signal Red
116 Caviar MB-Tex