You have it that multipliers just mean roll more dice?
I've always taken multipliers to mean exactly that: you multiply either [what's rolled] or [what's rolled plus bonuses]* by the multiplier given. Relevant in RAW both for backstrikes and for xxx-slayer weapons that multiply their damage against certain foes; also relevant for us in that our critical hit system is entirely based on damage multipliers.
* - 1e doesn't make it clear which it's supposed to be; we've always used the latter, meaning e.g. that a Ranger using a Giant-slayer (double damage against a Giant) can sometimes pull off some nice big damage numbers.
Page 27 of the PHB says "twice normal for the
weapon used per four levels of the Thief", but I agree that's vague. I started playing 1e not long before the 2e books came out, and there, they specified "the weapon's standard damage is multiplied....
then strength and magical weapon bonuses are added", so I've always played it like this to my memory.
What was actually intended for 1e Thieves is up in the air, but given how horribly treated they are overall, especially in the DMG, I'd be surprised if you were, in fact, intended to multiply bonuses, lol. I used to have a Wood Elf Thief with a girdle of giant strength who would have been a menace if they got to multiply their bonuses...well, that is, if I'd ever even tried to backstab anything. Which I didn't, because...story time!
A friend of mine had a ring of invisibility with the rare inaudibility feature, and I remember being insanely jealous of that item, since it allowed him virtually no chance of failure to actually backstab- or so we thought.
What actually happened was we encountered some "humanoid creatures in a dimly lit chamber" and he tried to sneak up on one. The DM chuckled as he happily described the Grimlocks detecting his presence by
smell and ripping his Thief to shreds.
Yeah, that was enough of a lesson for me.