But this just changes the question to "why is an ounce what it is".

OP asks "How could ancient people confirm and verify the mass of the pound? What was the benchmark?"

Thank Pieter; so the tldr is "a pound was arbitrarily decided; it wasn't, in general based on anything". Did I get that right?

@axsv177 The OED says a pound of 12 ounces had 5,760 grains, and a pound of 16 ounces has 7,000 grains. And I think those were legally the grains of specified plants.

@MAGolding Yeah, I get that. But I can't imagine the first decision about defining a pound as "Let's base weight on 5,760 grains". I can see "Let's base weight on the weight of a loaf of bread or a large apple". Perhaps 5,760 is the amount of wheat grains it takes to make a loaf of bread?

@axsvl77: Having made my own bread for many years, I can absolutely assure that no baker in history has ever counted the individual grains; then milled the wheat into flour; and then, as is always the case, adjusted by several tea or tablespoons for the right dough consistency according to the temperature and humidity of the day.

Agreed. Also, nobody has ever said "Get me a bag of rocks that ways the equivalent of 57,600 grains"

@Pieter Geerkens : Yes, but 5760 evenly divides by 12 into 480, i.e. a whole number of grains to the ounce, so my guess is that it provides a divisible number that approximates the weight of some object, such as perhaps, a loaf of bread as suggested. The question is, what exactly was that object, or is that unavailable from existing historical evidence?

(Of course, it could also be that there is no specific object - that the number was chosen because it divides into 12s and also happens to be "just the right" magnitude for many small but substantial objects of the time.)

And how do you count 7608. 9496 grains?

That it was not an exact mass unit is obvious once you think about what it took to create one, and how hard it was to do -- internationally the kilogram was only standardized 130 years ago, well into the industrial age.

Systems based on a number divisible by 2, 3, 4, and 12 is an ancient ideal. Not sure why, but it's kind of common.

"'libra pondo' which we translate into modern English as 'pound by weight'" -- I don't know Latin grammar, but I would think it makes more sense to mean "weight by pound", "libra" still being the unit of weight equal to the "pound". Perhaps the origin of the unit can be found by looking into the etymology of "libra". I can only find that it seems to originate from the Greek "λιτρα".

@JoL libra pondo translates to "pound by weight" because libra translates to "pound" and pondo means "by weight" (it's related to "ponderous").

@phoog I must be experiencing some brain fog. Could you tell me a sentence that makes sense with "pound by weight"? I could interpret "weight by pound" using the definition of "by" of "with the use of" as in "weight with the use of [the unit] pound", as in "can you tell me the weight by pound as opposed to some other unit?". "Pound by weight" only makes sense to me if "pound"/"libra" refers to something other than a unit of weight, as in "do you sell X by weight or by piece?", but what definition of pound/libra could work for that?

@JoL: Translations are only rarely, and serendipitously, exact. Latin libra also meant any of "[a pair of] scales", "balance", or "level", and is the root of the name of the metric unit "litre" (or "liter" in the U.S.) for volume measurement. The Latin phrase "libra pondo" existed as a semantic unit, not as two independent words, that when combined meant "pound by weight".

I also think to conceive of the missing part here as "where does the ounce come from, then?" is not right - what's missing is "where does 5760 or 7000 grains come from?"

@PieterGeerkens IANAL and I honestly do not know, that's why I was asking. Fair Use is notoriously hard to determine where exactly the boundaries are, so I don't know whether including the complete text-image of a single definition of a single word counts or not. I also don't know whether UK or US rules and laws apply here (or both). US Work For Hire rules here would be 95 years, which has certainly passed.

@PieterGeerkens If I had the OED I would have checked it, but I do not.