The commodity that is most universally accepted represents abstract labor (which is identical as its concrete labor). So in this intermediate stage between barter and the settled full value-form (money), which we are discussing (roughly 7000 years of human history), e.g. the labor for cattle, salt, or copper, etc. would have been (closest to) abstract.
I think you're evading the issue. Suppose you're correct and there's a commodity M for which concrete labor is immediately social labor. The real issue is how do you reduce the different concrete labors (and labor times) producing any commodity A to the concrete and immediately abstract labor producing commodity M. There are varying labor times (productivities and intensities) for commodity A. Which one do you pick? I don't think you can say we simply calculate the arithmetic average because you don't know the input values – unless you suppose that these peasants kept records of labor times of all their "competitors" and then used arithmetic to calculate the average labor time. They'd have to do this for all commodities which they exchange. I think it's much more reasonable to suppose that it was hit and miss, with lots of "political", extra-economic intervention, habitual pricing and sheer theft/usury/racketeering going on (all of which fits well into the image of precapitalist societies we have).
(I'm disregarding here the fact that socially necessary labor time isn't simply average labor time – the quantities produced and the amounts the market can stomach also matter, so it's more like a "weighted average".)
They bother because there's already some division of labor, but not so complex that it becomes opaque. The point Engels made about fetishism is that it is caused by the usually foreign origin of precious metals, so that local people don't know how much labor went into it. Though I think there were also local mines where people did have an idea. In any case if it happens that A and B don't know what the other's product is worth (its SNLT), then they could still compare it to a third product that they do know, eg cattle, which acts as proto-measure of value (hence the supposed difficulties of barter are often overstated).
Correct me if I'm wrong, but you seem to suppose that somehow there was a division of labor that naturally led to development of the market, which the peasants willingly turned to as an "opportunity", as some little medieval or ancient NEPmen, which then led to more market expansion, the division of labor becoming more opaque etc. This is very much like the "commercialization" model in Smith and some marxists. I think this is very problematic, as shown by the research done by Brenner and his followers.
It's not forbidding exchange of different labors per se, just that the principle of equality should be applied (i.e. work of the same difficulty/intensity).
I think the text it's pretty clear on this:
"Just as two different types of labor may not be exchanged for one another, so too the same labor may not be exchanged for the same labor if they are done during different seasons."
The point is that they were already aware that different labors can have different intensities.
Surely that's a fact of basic experience which predates even class society.
The commodity that is most universally accepted represents abstract labor (which is identical as its concrete labor). So in this intermediate stage between barter and the settled full value-form (money), which we are discussing (roughly 7000 years of human history), e.g. the labor for cattle, salt, or copper, etc. would have been (closest to) abstract.
They bother because there's already some division of labor, but not so complex that it becomes opaque. The point Engels made about fetishism is that it is caused by the usually foreign origin of precious metals, so that local people don't know how much labor went into it. Though I think there were also local mines where people did have an idea. In any case if it happens that A and B don't know what the other's product is worth (its SNLT), then they could still compare it to a third product that they do know, eg cattle, which acts as proto-measure of value (hence the supposed difficulties of barter are often overstated).
It's not forbidding exchange of different labors per se, just that the principle of equality should be applied (i.e. work of the same difficulty/intensity).
The point is that they were already aware that different labors can have different intensities.