Miller,
I was off a year, it was 1892.
http://www.google.com/patents?id=t0V2AAA...nri+pieper+1891 It has to do with the lock up.
By 1892 he was producing the Modified Diana, the tubes and monobloc no longer had that step.
Here is a 1897 Sears ad showing the modified Diana and talks about screwing the barrels in. They also warn about imitation Pieper guns.
Here is the description from 1881 which makes no mention of threads:
http://www.google.com/patents?id=QiBMAAA...as_maxy_is=2008
Through the breech-piece a are bored two conical holes for receiving the two barrels b 1), which are turned or shaped conical—that is to say, thicker on their breech euds than on their muzzles—so as to fit accurately in the said conical holes of the breech-piece. The direction of these conical holes is such that when the barrels are introduced into the holes they meet or touch each other at their muzzle ends, at which extremity they are united by Iheir insertion into the double link c, which tits over the reduced or offsetparts of the barrel ends on the muzzle. On the under side of the barrels the breech - piece a has the extension a', running up the barrels a suitable or desired distance. Near the upper end it has the plug a3, extending up between the barrels and forming a point of attachment for the screw in upper strip. This serves to steady and hold the barrels, and keeps them at just the right distance apart. The open space between the two barrels & &, from the breech end to the double link c,is closed throughout its entire length by means of the two narrow strips d and e. The wider of these strips, d, is placed above the barrels &, forming the prolongation of the strip i of the breech-piece a, to which it is attached by the two screws / g, the other end of it being fastened to the double link c by the screw h. The narrower and smaller strip, e, is placed below the barrels a, and fastened there in a similar or in any convenient manner. Finally, the so-prepared barrels &, breech-piece «, double link c, and strips d e are brazed or soldered together.
It must be understood that the breech-piece a can be used, in combination with the double link c, on double-barreled guns, or only one of these devices may be used, the other or its equivalent then being constructed in another way.
Pete
