I think of the extra reloads as more of a quick charge kit that is used up. It has a canister of tibanna and quick charge battery recharger. These are not meant to replace proper weapon maintenance but an emergency, in field, quick fix. Due to the probably millions of different blaster types it is unlikely that these extra reloads are geared towards a particular model.

My making them a universal (quick reload) kit they are extremely versatile and explains the encumbrance value.

What Chewbacca was wearing was a physical representation of the spare clip talent... or the wookie equivalent of a Gucci handbag.