Gold Apollo lists AA, FWIW

~~https://web.archive.org/web/20240415091632/https://www.apollopagers.com/shop/gold-al-a25-alpha-numeric-pager/~~

Power from a single AA alkaline battery (plus lithium backup battery).

EDIT: The AP-900 uses AAA.

And yeah they absolutely are legit and are used all over the industries that still use pagers (hospitals are big big one in part because of their reliability and lack of EMI).

Another interesting point about them is that they are:

UL-certified for use in hazardous locations