The origin of "premium" as an adjective

The original meaning of premium is a reward given for some specific act or as an incentive; a prize. as per its etymology: Premium (n.) (Etymonline):

• c. 1600, "reward given for a specific act," from Latin praemium "reward, profit derived from booty,"

The OED cites early adjectival usage examples referring mainly to petrol especially in AmE:

Premium:

• (Passing into adj.) Of a commodity, etc., esp. petrol: superior in quality and therefore commanding a higher price; of a price: such as befits an article of superior quality; higher than usual. orig. U.S.

• 1928 National Petroleum News 24 Oct. 115 (Advt.), This is our anti-knock gasoline, a premium motor fuel.

While according to Etymonline its first usage was in 1925 referring to butter;

• Adjectival sense of "superior in quality" is first attested 1925, originally in reference to butter.

Questions;

What's the origin of premium as an adjective? Was butter given as gift/prize, from which the sense of "top quality"? Was its origin from AmE usage because of its common reference to petrol or was "premium butter" a BrE expression?

Solution 1:

As usual, it's a metaphor.
Premium in the sense of 'reward given for a specific act' is synonymous with prize.
Both words refer to a Contest frame with multiple entries, judgements, and prizes awarded.
And then the winners get to brag about their prizes (even if they're imaginary).

So premium butter is the same construction as Blue Ribbon Beer or New York Times Bestseller 'The Joy Luck Club'. Essentially the prize/premium name is used as an appositive, and then a modifier, to the following noun, which may be a proper noun.

As to when a noun that modifies another noun becomes an adjective, the Academy decides.