Are you positive on that number? I got the same number from Jamie @ Subaru Genuine Parts and she said it was a top o-ring and that the bottoms are not available. I've also seen other threads with numbers such as (but not sure which is which):
16698AA050
16698AA060
16698AA080
16698AA100
16698AA110
Any insight? I'm missing the same o-ring for my 07 STI injectors...
Thanks!