Comment by fencepost
Thinner (less durable, but who cares?) plastic shell to free up space for explosives, but would likely be obvious if someone opened it - which might be a common thing if these were being used as remote triggering devices.
If they were using a AA battery, replace the battery with something that provides you space to work (e.g. put in a AAAA or button cell that would provide appropriate power but lower capacity) because you don't really care if the battery life drops from months to weeks.
I can easily envision a scenario that would preemptively “explain” why the pagers are internally different from past models:
Supplier: “Hey, we’ve got a refreshed model of the pager you wanted to buy in bulk. Interested?”
Buyer: “I don’t know, how do they work?”
Supplier: “Same as the other ones, minus a bit less plastic protection. With the weight savings they’ve added a new hardened receiver that’s supposedly more secure and will keep communications private. Also, they are 50% cheaper per unit…”
Buyer: “Say no more. We’ll take them.”