Industrial non-uniformity generates privileged closed-shop labour aristocracies.

Evidence

Since there has never functioned and does not function any law of perfect parity of systems and production and work methods valid for all firms in a specific branch of industry, it follows that every firm is, to a greater or less degree, “unique” and will form a labour force with qualifications proper to its own particular requirements. Little manufacturing and working secrets, or “fiddles”, practised by this labour force, which in themselves seem insignificant, can, when repeated an infinite number of times, assume immense economic importance. … There has to be a skilled labour force which is permanently available (which does not absent itself from the place of work) to deal with the minimum of seasonal or other work, and this leads to the formation of a kind of closed shop with high wages and other privileges, opposed to the mass of “casual” workers. — Antonio Gramsci, Selections from the Prison Notebooks, 1147