The Parker Gun Co records do exist.
The gun in question can be searched and how it left Meriden can usually be found.
There are some ser#'s that are missing and not retrievable in the records, but generally complete.
Parker did make a few 'Cape Guns' . One bbl a standard shot bbl, the other a rifle caliber.
There are a few 'drillings' around as well with the standard SxS shot bbls and a small bore rifle bbl planted on top of the shotgun bbl's.
The Factory records of those Drillings AFAIK usually shows that the gun left the Factory as a standard SxS shotgun. The conversion to the Drilling happening aftermarket.
There was a company in NewHaven Ct in operation at the same time as Parker was up and running in Meriden Ct.
That other company was making Auxillery rifle Bbl inserts.
I think Parker used that company to supply rifle cal inserts for special order Parker customers that wanted an CF rifle cal insert fitted as an option along with their Parker SxS shotgun.
AFAIK, when the Parker Story went to press (1996??) there where no known examples of a true Parker Factory built double rifle.
Maybe since that time one or more has come to the surface. I don't know.
It is not all that difficult to build aa DR on a SxS shotgun action. It's done all the time.
A 45-70 or 45-90 is a common caliber to choose for a large ga shotgun frame both for reducing weight (big hole,,less weight) and keeping psi down.
Using the orig top rib of part of it keeps the orig bbl address roll marking in tact as well as the signiture Parker top rib matting pattern.
Though both the roll marking address (there's more than one style) and the matting can be done with new roll dies and CNC mill cut. An expensive alternative.
Pics would certainly help.
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A trip over to the Parker Gun Collectors Forum with the info, pics and questions will draw a flury of comments.
Parker Gun Collectors Association Forums