Weight Weenies

That's what I did. I think it's silly that they offered both a non S2 version and an S2 version. From a business and Sku stocking standpoint it's gotta be much more efficient to have just the S2 version and then have the option of using the support arm or just leaving it off. I actually only had the option of ordering the S2 version for my C60, but received the non S2 version. I didn't care since I knew I wouldn't need the support arm anyway, and without the bolt it's a little cleaner looking.

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@sleepless, even against the rivet, that should provide some stabilization

Point is, it is redundant on @Sleepless's frame (C60) because the brazeon tab and the tube it is riveted to has no significant flex in that particular spot. The support arm is only relevant on frames where there is excessive flex in that area. You'll see that more on aero type frames where the frame surface that the brazeon is attached to is often a lot flatter and inherently more likely to flex with the force that a front derailleur exerts on it. And derailleur tabs themselves are not all created equal either.

Found this thread because I was confused what was the S2 variant on the Campy 2015+ front derailleurs and wanted to share some pictures for posterity.

Campagnolo sells just the tab with the model number "FD-SR006". Pulled this image from an eBay listing.

Love the product name of "front derailleur stiffness increaser":
Otherwise the front derailleur variant with the S2 tab should be FD15-CH2BS2 as seen in the sticker in an actual product with the tab.
The non-S2 variant seems to break from the usual naming convention of [component][year]-[level][variants] with FD15-11CH2SP.