I've owned some that didn't appear to be vented but when closely examined I found they were. I too referred to the vents as blow out plugs...while in reality they don't blow out nor were they designed to blow out.

If you study the history of percussion guns you'll find that the flint lockers claimed their guns shot harder because the powder could draw air for ignition. In response to this claim the percussion cap advocates put in the vents. In later and lower grade guns you don't see the vents.