I thought this was going to be the final installment of my review of Marx's writing on socially necessary labour time but then I discovered, as I was going through my posts that I haven't done the draft "chapter six" that contains the fascinating discussion of formal and real subsumption. So there will be either one or two mores posts. Yay!!
An index page of all the posts so far -- both numbered and unnumbered -- is here.
Chapter 38, "Differential rent: general remarks," contains an interested recapitulation of the relationship between market price and value, specifying the "socially necessary quantity of commodity varieties" as part of the determination of socially necessary labour time:
It is in general in the form of the market-price, and, furthermore, in the form of the regulating market-price, or market-price of production, that the nature of the value of commodities asserts itself, its determination not by the labour-time necessary in the case of any individual producer for the production of a certain quantity of commodities, or of some individual commodity, but by the socially necessary labour-time; that is, by the labour-time, required for the production of the socially necessary total quantity of commodity varieties on the market under the existing average conditions of social production.
This isn't a distinction I have encountered before and a search of the collected works turns up no similar phrase. It would appear to refer to the fact that a significant quantity of variety of use values would need to be present on the market for generalized exchange based on abstract values to occur. Later in chapter 38, Marx explained how monopoly access to a waterfall, for example, would enable a capitalist to avoid the cancellation of surplus profit through other capitals introducing the same technology. "It is by no means within the power of capital to call into existence this natural premise for a greater productivity of labour in the same manner as any capital may transform water into steam."
Chapter 49, "Concerning the analysis of the process of production," reiterates that "Profit (profit of enterprise plus interest) and rent are nothing but peculiar forms assumed by particular parts of the surplus-value of commodities." Marx discussed socially necessary labour time here only in connection with explanation that surplus value may not be entirely realized and that the apportionment of value into wages, profit and rent is simply a capitalist form of expression for "the measure of socially necessary labour contained in a commodity."