info re multiple licensees is from Campbell's book..
h&r's exclusive license expired in 1885...
OK Ed, so you're saying that maybe Shaefer built this gun under license from A&D after 1885, but for some reason, he decided to top it off with a Harrington & Richardson buttplate?
If it wasn't for that little detail, I'd say perhaps your theory is a possibility. Something like the Lefever clones built on Lefever frames by J.A. Prechtel. But Prechtel didn't use buttplates with the Lefever logo. The guns he built had no Lefever markings. The frames had Prechtel's name. And he built more than one.
So it would bolster your thought if you could show multiple A&D actions with Scheafer's name.
Your idea about the origin of this gun is interesting, but very unlikely. The idea isn't nearly as irrational as Princess SKB's imaginary top lever cocking SxS hammergun. And there is no shame in selling a scarce H&R double that was rebarreled to 12 gauge. It's still interesting to examine and theorize how it came to be.