But there is this vital difference: the critical velocity c which occurs in these formulae has now no connexion whatever with light or with any other fact of the physical field (in distinction from the extensional structure of events).

It simply marks the fact that our congruence determination embraces both times and spaces in one universal system, and therefore if two arbitrary units are chosen, one for all spaces and one for all times, their ratio will be a velocity which is a fundamental property of nature expressing the fact that times and spaces are really comparable.
The physical properties of nature are expressed in terms of material objects (electrons, etc.).

The physical character of an event arises from the fact that it belongs to the field of the whole complex of such objects.

From another point of view we can say that these objects are nothing else than our way of expressing the mutual correlation of the physical characters of events.
The spatio-temporal measurableness of nature arises from (i) the relation of extension between events, and (ii) the stratified character of nature arising from each of the alternative time-systems, and (iii) rest and motion, as exhibited in the relations of finite events to time-systems.

None of these sources of measurement depend on the physical characters of finite events as exhibited by the situated objects.