[torstenvl]

As others have said, there's the pluralization of "peanut[s]" to distinguish between the two. This is a useful feature of English: the adjective-like role of a noun in a complex noun phrase is (almost?) always singular.

    - Computer engineer
    - NOT computers* engineer

    - Toothbrush
    - NOT teethbrush*

    - Foot doctor
    - NOT feet* doctor

    - Alarm clock
    - NOT alarms* clock, even when it supports multiple alarms!

Additionally, there's phrasal intonation. If the intonation and stress decrease throughout the phrase, it's a single item. If the intonation and stress reset for "butter," then it's a new item.

    - 'PEA ,Nut but ter
    - 'PEA nut 'BUT ter

[fooker]

Proudfeet!

[sakebomb]

Alexa, please tell me about the...

Attorneys general Senators elect

Ahhhhhhh!

[ksenzee]

The difference is that these are phrases with adjectives, not nouns being used adjectivally.

[a1369209993]

"Attorney generals" is a noun phrase (admittedly of questionable adjectivity). "Attorneys general" is a blind idiot translation of a phrase in a language with different grammatical rules (Latin, IIRC).

[gowld]

"Attorney" is a noun. "General" as used here is an adjective. It's unusual in that the adjective follows the noun without a hyphen, but it's common enough, and it's where prepositional phrases are seen, like "Big man on campus" and "powers that be".

Did ancient Romans have attorneys general?