There is nothing significant in these chapters regarding socially necessary labour time.
Chapter 12, the concept of relative surplus value, would be a good place for Marx to tell readers that socially necessary labour time, per capital, entails the production of a relative surplus population. Instead, we get only an anodyne definition of the productiveness of labour:
By increase in the productiveness of labour, we mean, generally, an alteration in the labour-process, of such a kind as to shorten the labour-time socially necessary for the production of a commodity, and to endow a given quantity of labour with the power of producing a greater quantity of use value.
Chapter 13, co-operation, gives a slightly confusing discussion of what happens when one worker exceeds the time socially necessary to produce a commodity:
If one workman required considerably more time for the production of a commodity than is socially necessary, the duration of the necessary labour-time would, in his case, sensibly deviate from the labour-time socially necessary on an average; and consequently, his labour would not count as average labour, nor his labour power as average labour-power.
Typically "one worker" does not produce a commodity, especially if we are talking about "co-operation." The "duration of the necessary labour-time" refers, in this sentence to the portion of the working day that the worker is compensated for while the labour-time socially necessary refers to the average time required to produce the commodity. Those two durations would "deviate" from each other in any event, since they refer to times required for different purposes - reproduction of labour power and production of commodities, respectively. Marx's purpose here is to explain the rationale for assuming a fixed minimum of labour efficiency.
Chapter 14, division of labour and manufacture, explains that the rule of socially necessary labour time is enforced by competition, since "each single producer is obliged to sell his commodity at its market-price."
Chapter 21, piece wages, explains the piece-work is no different from hourly work in that the piece rates are established by observation of how long, on average, it takes to produce each piece, "Only the working-time which is embodied in a quantum of commodities determined beforehand, and experimentally fixed, counts as socially necessary working-time, and is paid as such."
Chapter 22, national differences in wages, relates wage differentials between countries to differences in the average intensity of labour in each country: "In every country there is a certain average intensity of labour below which the labour for the production of a commodity requires more than the socially necessary time, and therefore does not reckon as labour of normal quality."
I skipped chapter 15 in this post because I am going to include it with chapter 25 in a forthcoming post.