IMHO, the sale favored the buyer & I hope someone here got it.
Also, let me clarify what might have come across a bit snarky (unintentionally) - when I say the discussion's silly..I mean that it doesn't require an overabundance of hyper-technical dialogue. It's simple..drop a scale into the chamber, make sure it's edge is pushed down firmly against the chamber wall and take it to the forcing cone. If it's basically 3" to the forcing cone, have at it with all the 3" shells you wish to fire. Or, limit your ammo choice to 2-3/4" and never, ever worry about it. There's arguably an excessive margin of safety now built into the most current standard and I'd guess that likely has more to do with attorney's than anything else. That's not to say it's not interesting and thanks to Johnny for sharing the info, along with his perspective that isn't exactly right but isn't exactly wrong either. Now Johnny will show up and slap a revision on us from 1982 that shows chamber lengths were increased back then and we've all got it wrong..
I've not checked my 12ga, but I will b/c I can't recall how it's marked. I think I recall John Allen sharing the factory all but insisted on multiples of 25 when accepting an order so their internal numbers made sense for a production run. That fact would likely explain inconsistencies in chamber length across the 12ga's, I would think, no?