The tank would be 65 litre capacity, the code refers to 50 litres of fuel ordered with the car, one would think that a new car would come with a full tank of fuel, but even as of today i think cars only come with enough fuel to get off of the forecourt
The option code list in the tech manual relates to codes applicable to the W113, as it should, but there is a much more comprehensive list published, of which Pawel attached an extract, which includes all models, up to date. It's not always clear which codes apply to which models, so it's good that you've proved 317 seems to apply to early 230's
If you look at the codes in the tech manual code 663 is also for 50 litres of fuel, so what's the difference, why two different codes?