Good photo of a vent, Daryl.
According to Col Peter Hawker, when the percussion system was first employed, it was thought the pressure would be too high , as there was no touchhole to let some of the pressure out, so a vent was used to reduce pressure a little.
As these original vents were so small, rain entering was never a problem.
Some of mine have these vents, but most appear clogged .
The vent t was not a dumb idea, it was the product of logical thought. The flint-gun had been in use for hundreds of years, and making a gun with no 'pressure valve" seemed wrong.
This, plus the fact that original Early caps were powerful enough to move the charge prior to proper ignition, and we can see the (logical) reasons behind the vent.
Best,
R.
PS,
Couple of mine.