There is a fundamental distinction between the metrical properties of point-tracks and rects.

But in the fourth assumption this fundamental distinction vanishes.
Neither the third nor the fourth assumption can agree with experience unless we assume that the velocity c of the third assumption, and the velocity h of the fourth assumption, are extremely large compared to the velocities of ordinary experience.

If this be the case the formulae of both assumptions will obviously reduce to a close approximation to the formulae of the second assumption which are the ordinary formulae of dynamical textbooks.

For the sake of a name, I will call these textbook formulae the 'orthodox' formulae.
There can be no question as to the general approximate correctness of the orthodox formulae.

It would be merely silly to raise doubts on this point.