Help with a hammerless double barrel teardown

[Chris Wren]

I recently inherited a Parker double barrel hammerhead shotgun. Serial number puts it built in 1918 (1820xx). I am trying to inspect and figure out all of the specifics of this unit, but I cannot separate the barrel from the breach. The breach opens, and ejects shells, but unlike my Remington double barrel, the barrels do not rotate far enough to separate. It feels like it hits a stop. I have removed the grip on the front. If you look at the technical page, it feels like the pivot pin infront of "check hook T" and "pin E" (looking at figure 8) is not allowing the barrels to rotate far enough to clear Check Hook T from Pin E.

I have attached a photo of how far I can open the shotgun.

Any ideas what could be stopping me from separating the barrels from the breach? Thank you for your help?