The pattern is one that Robeson did make and the handle material and shield are quite common to both Robeson and Terrier knives, putting it, at least consistent with a 1910 to 1916 time period and probably beyond.
Dewey and Lavonna Ferguson had limited access to Robeson tang stamps and they broke them down as best they could, at the time. When, in actuality, there were several tang stamps that were used between 1901 and 1948.
Tom Kalcevic, in Knives Can Talk!, dates that stamp 1911 to 1921. I use Tom's dates, but I do not guarantee them to be totally accurate.
I do not know exactly when that handle attachment technique was initiated, but the patent information for it is available somewhere on the internet, as there was a protracted discussion re' it in Bernard Levine's Whut-Izzit? column in KnifeWorldsome years ago.
I have seen stabber jacks by Robeson, but all I have seen had two blades. I do not recall any pattern numbers from the ones I've seen and I do not recognize the handle-die shape number 518, as on your knife.
Did the cutler simply put the wrong stabber jack blade in the knife or has the secondary pen been broken and subsequently removed?
I don't know.
The knife is, most likely, a Robeson, but whether or not it left the factory as it is now or not is a question I cannot answer.
Here is a bone handled two blade coke bottle stabber jack photo that I took off the internet, but there was no published info on the pattern number, nor a photo of the back of the blades. Given that impressive long pull, that knife probably pre-dates yours.
Here, also, is a photo of a CARPENTER'S CHOICE three blade with the patented handle attachment technique. Given the lack of a bottom bolster on your knife, it needed a pin in that location.
DE OPPRESSO LIBER
"...Men may spurn our appeals, reject our message, oppose our arguments, despise our persons ___but they are helpless against our prayers. "