Pitts, this is getting more interesting. In your third picture, the 'wedding band', the engraving around the tubes at the breech end of the barrel, does not correspond to any V grade engraving pattern. This has me wondering, if, as you say, the gun was originally a 28 ga., and the barrels were damaged, and re-tubed, as evidenced by the ring engraved around the tubes, forward of the hinge pin. If that was the case, does the serial number stamped on the side of the barrel lug match the number stamped on the water table and trigger guard?
Since the 'wedding band is characteristic of at least a D grade, perhaps the original barrels were lost, and another set of donor 0 frame barrels were sleeved to 28ga. It is common for barrels to be re-stamped to match the gun's SN when replacements are fitted.

A faint seam line around the rim recess where the doner tubes were inserted as an interference fit to overbored chambers..

This is quite the education.

https://imgur.com/QAecFRH
https://imgur.com/18RyOVp
https://imgur.com/18RyOVp

Is that a research letter from the Parker Gun Collector's Association?

It is and I just posted it. in the link above.

I cannot see pictures real clear for some reason..However is there a seam line at 1:00 o'clock on the right barrel just outside the rim recess ??

It's easy to see the boring line around the breach chambers. The old barrels ribs removed, barrels cut off & breach bored out & sleeved barrels inserted, soldered & rechambered.

Yes, I can see it well, on both barrels. Also, the joint between old and new tubes is visible. It is the right hand edge of the engraved “chain”.

Yes, & also the tubes weren't centered on the boring. They are pencil thin at the breech from about 3 o'clock to 6 o'clock.