I'm no expert, but I've just unscrewed the post off this SID a bit and it looks more suitable than the one you're using. No second step? I don't see the purpose of the round bit on yours unless it sits within the frame.
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Yes, but the SID's second (round) step is less than 1mm, whereas the one on eBay is about 3mm and yours looks longer still. It's the fact that your second step is longer than the spur off the tensioner that's your problem, not the fact that it's there at all. 1mm doesn't stop it engaging.
Definitely the wrong bosses. I had a frame that had the extra wide part as yours do, but on my frame this screwed right inside the frame and sat flush.