Bruce,
Thanks for the great info. It sounds like Parker sighting bead holes tap into the "void" between the ribs. With most other brands of doubles, the bead hole is a dead end into the solder. The Parker bead hole alone would probably suffice to allow the quickly expanding gas to vent out when the barrels are dropped into boiling water. I don't have much Parker bluing experience, but I can tell you from rust bluing other brands that the gases expand violently and really bubble out that vent hole with force when the barrels are dropped into boiling water. I would be hesitant to boil any set of soft soldered barrels, no matter how well made, without a vent hole of some sorts to release the pressure on the ribs.
Also, it is generally not considered enough to only drain the water out of the void between the barrels because moisture and rusting solution salts can be left behind to cause problems. Most pros advise that the void be flushed out with a special water displacing oil, such as that sold by Brownell's, which isn't cheap. Its thinner than water and can easily be injected into the smallest hole with a large hypo needle. After being sloshed around thoroughly in the void, it is then poured out through the vent hole.
Speaking of vent holes in ribs, the older higher grade LC Smiths had the typical vent hole behind the forearm lug, but they threaded it and filled the hole with a small flush fitting plug screw, which looks quite nice.
The practice of drilling a vent hole in the bottom rib of a Parker for rebluing is so common that I have read several articles over the years advising that originality of bluing on Parkers can be easily verified by simplying making sure that there is no vent hole. That is obviously not true, as we can see with this subject D grade. But it does seem to indicate that most gunsmiths mistakenly think the new vent hole is essential.
Perhaps one of the pros like Brad B would be willing to post a thread and explain to us the ins and outs of rebluing Parker barrels without drilling additional holes. I'm sure that many of us would find it fascinating even if we have no intention of ever rust bluing a set of barrels ourselves.
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