I think we spend far too much time upon minutia. Ditto the bit about a lug that penetrates the frame.
I used to buy into the whole "sidelocks are superior" er, "myth", I think is what you mean, but I'm pretty sure Zutz, I believe, said that fit was more important than hype about gun types.
Truer words were never penned.
A bird killed with a sidelock isn't any deader than one killed with a boxlock. Or, hell, a pump, for that matter.

Best,
Ted