It is written ambiguously, thus 1e rules lawyer arguments to be made.
The girdle text says outright the two stack with any hammer, the gauntlets suggest it is desirable in combo with a girdle and any hurled weapon (so spears, axes, daggers, darts too) but does not state why, and of course the Hammer of Thunderbolts does not talk about other weapons.
Not really (to the bolded part). This is something that is lost on a lot of readers today, but Gygax used
q.v. in a specific context- when he used it, he meant that you were to cross reference what he just wrote with another specific rule.
So the part of the girdle text says "
and magic war hammers (q.v.)." which means that you have to find the specific rules in the text for magic war hammers, which exists- but only in one place. That's why there is a "q.v." there.
Side note- q.v. is a signal (like cf.) and is Latin for quod vide, meaning "on this (matter, issue) go see" So it's a way for an author to refer to something else in the book that deals with the same subject matter, and Gygax is a big fan of it. It's also an easy way to avoid footnotes, repeating material, and, um, showing your work. Ahem.
Here's some examples of how he uses it:
DMG at 44:
Erase: This spell might be useful against a glyph of warding (q.v.).
So you look ....
DMG at 41:
Glyph Of Warding: If a cleric is on hand to determine that a glyph of warding is certainly in existence, an associated magic-user can thereafter use an erase spell and possibly (50% + 2%/level of the m-u) remove the glyph.
See!
Once you start to realize that Gygax is using this specifically for cross-referencing rules, you can't unsee it. The PHB, alone, uses it, IIRC, over 150 times.