Those are neat flasks, both top one and Mike's. The cap-feature is certainly "neat", but I've found that a capper worn around the neck works better for me, along with a flask. I carry my slugs or balls in the "civil war" cap pouch, so to reload in the field, I charge the cylinder with the flask, grab a slug from the pouch, load that, then only need to hit it with the capper. Fishing out six caps (or one or two for a partial reload) one by one, or pouring them into your hand to cap by hand, would be a slow process, and could result in the dropsies. The concept/practice of having six chunks of lead in the flask is good.
Or in other words, having a place to store spare caps is fine, but I'd still use a capper. I just carry a tin of caps in one of my "ranger vest" pockets, in order to recharge the capper.
I often hunt, hike, trek, explore, and play map&compass in some pretty wild country and most often carry a cap-n-ball revolver, and that's what works best for me so far. As wolves have become a threat where I roam (first attack on humans last October) I've put some thought into it. !!!
Why don't I carry a modern pistol? Beats me. Can't explain it.
I made something similar about 25 yrs ago using a Walker flask. I shot conicals and wanted the bullet magazine to hold 6 instead of 5. I added brass tube to the existing tube and fixed that problem. Then I decided I might as well add a cap magazine. So I did. Here's a pic.
