Nope, you are missing some of what has already been established. In the case of a magazine, if it is not pinned/rivetted/crimped/etc so as to hold only 5 rounds, then the magazine itself becomes a Prohibited Device. How you fill the mag is irrelevant - it is the capacity which determines legal or prohib status for a detachable box magazine feeding device. Even empty, if the mag is capable of holding 6 or more rounds it is prohib, period. The age of the firearm for which the mag is intended is irrelevant.
The questions surrounding disintegrating-link ammo are different. Legality is apparently defined by the number of consecutive live rounds that are linked together, and the era of the firearm for which the links are intended. To whit, any disintegrating link designed for a post-WW2 firearm is limited to 5 consecutive live rounds. My question is whether the insertion of a dummy 6th linked round "breaks the count", such that another 5 linked live rounds can follow....