A different theoretical position on heterogenous labour is presented in my
book, The Basic Theory of Capitalism (Macmillan, Barnes and Noble, 1988)
6.1. It contains a critique of the type of interpretation which you are
concerned. The grading of wages could be done by various reasons for
managerial convenience to control workers as well as to enable reproduction
of certain sorts of skills and educated capacity. They may not closely
related to intensity or the amount of labour expenditure of workers. Can you
believe that workers increase their labour output annualy in the seniority
escaslation wage system for regular workers in Japanese big businesses? In
many cases the individual personal contribution to outputs in jointly
working places may not be separable.
All the best,
Makoto Itoh