Ah, the pyramid formation standard numbering system up to the 1940’s :-)
Yup, that’s where the numbering came from in countries that played this formation.
The gospel according to Wikipedia is an interesting read, although you should never take it that it’s always factually correct.
https://en.m.wikipedia.org/wiki/Squad_number_(association_football).
In the dispute between no’s 4 and 6 my own interesting theory is that 4 was usually right footed and 6 left footed.
4Rf def plays to the right of no 5 def with 6Lf DM in front, whereas 6Lf def plays to left of no. 5 def with 4Rf DM in front. If 4,5,6 all right foot (not many all leftie combinations) it was probably left to preference.
If one player got injured the replacement in the next game just took over the shirt number of the injured player*
*In saying this buzzer went from 7 to 9 and Frannie from 10 to 9 when they started as centre forward, but, I’m talking about 4,5 & 6 where they would usually keep the same shirt number.