If only... but alas, no hidden arm.

I've just checked the old chainring (pulled it out of the recycling bin), and the tab is oriented differently, it's between the mounting bolts, so it goes behind the crank arm when mounted. Interestingly, the old chainring's tab can be removed (small torx screw) and there's an alternate mounting hole for it inline with a mounting bolt, so that chainring could have the tab moved to accommodate either style crank.

Maybe this post can help someone in the future: if you get new 135 BCD campy chainrings, and go from 52 to 53 teeth, the little tab that goes behind the crank arm may not be in the same place on the chainring, and you may not be able to move the tab.

I suppose I could drill a hole in the new chainring and use the old chainring's tab... I'll save the old chainring in case I feel frisky this weekend.