Since several of you offered help in my period of frustration, my thanks, and I thought you might be interested in how the problem has been resolved.
According to the DGJ - Furnish / Nelson article, my gun is a Model 1877, the most common variation, and is as carefully described in the 1878 Fox manual, and the November 1877 Patent. So nothing new here except to verify what I thought that I already knew.
Therefore, I did what I was trying to avoid, made a 3/16 inch diameter trepanning tool, located the front forestock screw, drilled it out and removed the wood. As I feared, there was a piece of quite old, folded cloth, in the cavity beneath the barrel pivot bolt. Since this material was jammed around and under the head, it did not have clearance to pass through the enlarged hole ("keyhole") in the forestock plate, for barrel removal. Upon removal of the cloth and fragments, the barrels came off easily.
The third part of the extractor, that I call the "sliding block", was also in backwards. Therefore the extractor could not be replaced properly, and in fact the gun would not have closed. This explains why those extractor parts are missing!
I will refrain from commenting on the talents of the last person to work on this gun!
