Hi,
I recently bought one myself for my S1 and to be honest i didnt have a clue which one to buy so i just rung them in the end and ordered it over the phone by just asking for a sump plug for a S1.

Hope this helps..

BTW.
I found it a bit of a pain to fit as every time i got it anywhere near the sump it just stuck to it rather than going in the hole. Took me quite awhile which was rather annoying.