General points: 1) Interest is a product of a social relation, and so 2) interest bearing capital is about a relation of quantities – a ration between a principle as a given value, and itself as self valourising value.
Think Australian small business, where managers of capital (or owners of borrowed credit) have their profits defined by interest rates, as such they are compelled to intensify labour processes (see chapter 23, p505 for more on the general disciplining character of capital). In that way the most irrational form of capital comes to structure material reality. “The fetish character of capital and the representation of this capital fetish is not complete. In M-M’ we have the irrational form of capital” (p516). And later, “But if surplus value is conceived in the irrational form of interest, the limit is only quantitative, and beggars all fantasy” (p523).
The juiciest part of this chapter, I think, is in the sections on the length of production periods and the appearance of them. Marx raises the question of the “total working day” (see page 523) – social conflict determines length of working day, but there is a physical limit to the total social labour can be exploited. Here, perhaps it would be useful to think about current attacks on welfare, the NT intervention and other attempts to force people into certain forms of labour.