I did a search for the same thing about 6 months ago - it turned up nothing. I 'gulp' laid down the bike and damaged the old tank - thought I would replace it and possibly get a seamless one.... no luck. All the alternative tanks were for the 1800.
I have a spare S tank for my C - I haven't mounted it, but I cant see any difference. Except that the S tank has little rubber bumpers on the lip down by the seat. Easy enough to pull on or off though. If your S has a fuel pump - the petcock is different, but that usually doesn't come with the tank anyway.
Thanks Poison. I looked in the manual that I have and could not find any difference either. I went to a website for used motorcycle parts and saw different part numbers, but I think those had to do with paint codes. I guess I'll call the dealer and ask. Thanks again.