The trickiest part is keeping the lever out of sight while still letting it work. I’d start with a 15th‑century German “Kappelschiene” cuff – it’s basically a chain‑mail sleeve that already has a small iron bar that can slide. The bar was used to hold a dagger or small sword in place, so the mechanics are already there. You can lengthen the bar a few centimeters, tuck a tiny cam into the cuff, and add a spring so the lever snaps back when you let go. It looks like plain mail but you can lock a blade in a single flick, just like a secret courier would want. The only thing to tweak is the seam where the bar meets the cuff – hide it behind a flap of leather that blends into the mail pattern. That keeps the look clean while giving the covert edge.

I love the vellum idea—light, quiet, and it’ll soften that click. Just make sure the vellum’s edge is trimmed to the exact contour of the chain; any burr will show up when you flex the cuff. Also, a touch of beeswax on the bar’s tip will keep it from scratching the mail, and the wax will help the cam slide smoothly. Then you’ll have a covert dagger holder that feels as natural as a normal chainmail sleeve.

Just a quick sanity check: the vellum’s weight can be a silent killer if it sags and pulls on the cam. I’d cut a thin strip, fold it in half, and stitch the two ends together—sort of a hidden zipper. That way the vellum stays taut, the bar stays in place, and the whole thing keeps the whisper‑quiet seal even when the wearer twists the hand. And remember, the chainlink spacing matters—if the links are too wide, the lever will wiggle; too tight, and the dagger will jam. Balance is key.