Judging by the shape of the recess in the breech face it looks like the barrel extension would have a rounded end to fit the shape of the recess with a flat top (or slightly angled side to side) for the third locking lug / cam to engage. Try doing a cast to get an idea of the shape. Also, if you look at the height top to bottom, it is quite large, so the extractors probably wrap completely aroung the barrel extension lug with a rectangular hole in the extractors so they slide over the extension rather than slide along the bottom. I hope this makes sense. Here's a pic I found in a quick google search which shows what I'm suggesting.

This link shows a couple of other styles of hidden third bite.

http://www.hallowellco.com/concealed_third_fastener.htm

From what I've been able to work out, these hidden third bites need to be machined into the ends of the barrels (for shoelumps) or monoblock rather that being brazed on as part of the top extension like a greener cross bolt or dolls head extension.