It would be impossible to show a listing for comparison because here again what we have is a Bob Cash upgrade. This is a composed pair and hence has no collector value. The short tang adds another angle. $12,500 is a collector price. Good luck realizing the $12,5k. I’ll eat crow if you get it.
It’s not. More like $3-4,000, maybe even less than that.
So in essence (you back pedaling gasbag),
you're saying that if you take a stunning, field choked, 28 inch barreled 1967 RKST .410 bore (easily a $6,000 gun)
and have the "Best in the Business" Art Isaacson fit a proper Long Tang and add a complete set of impossible to find
Belgian Browning Factory 28 gauge, 28", field choked barrels (complete with its own fore end),
that I have devalued the original gun by 50% and that the extra barrel set has exactly zero value.
You really are stupid.