That's the part that always bothered me.
They did later do rebuilds of 15 rd mags with the open back followers (hold open) installed. I still consider a hold open more like the FN or M14have, M16...so you replace the mag and snap the action shut. The one for the M1 carbine is more like a warning that you're out, so try something else.
I'd heard this before. It shows men can't feel recoil, even heavy recoil of those muskets under stress. Guess they just forgot the alcan cap. imagine remembering after several ball/charges were loaded?