Thanks for the link. I think I see what this is now, the "internet archive." I clicked on title and it went back to their homepage on this book. I've found some stuff in the IA before, but at first didn't recognize the format at the level it was linked from your post.
To answer BoomJ earlier question re friction primer, there's a hole near the top of the breechblock for it. The flame goes thru an "L" shaped path to get to the front of the breechblock and into the center of the rear of the chamber.
One thing that is still a bit mysterious is why some of these Navy rifled howitzers have the vent in a "normal" position which is on top of the barrel, and some (mine anyway) have the vent at the top rear of the breechblock. Anyone know?
BTW there were a few different models of this weapon, the 350 lb. steel tube as shown in my video, and a 500 steel tube version. In our collection here there is one of the 350 lb. models as shown in the video, and two of the 500 lb. models. Both of the 500 lb. models have the vent on top of the barrel. The Washington Navy Yard also produced a few bronze-barrelled versions of the 350 lb. model, and I seem to recall, a few in the 500 lb. as well. I've seen a couple of the bronze 350 lbs. models, but none of the bronze 500 lb. examples.