Some teams had 4 and 5 at the back, others had 5 and 6. It comes from when 2-3-5 (known at the time as 'the pyramid' rather than numerically) gave way to the W-M formation, which then gave way to 4-2-4 and then 4-4-2.
Players were numbered:
11 10 9 8 7
6 5 4
3 2
1
To begin with, 2 and 3 were full-backs, 4, 5 and 6 were half-backs. The middle one (the centre-half, a term still occassionally used to refer to central defenders) was pulled to the back line by Herbert Chapman when the offside law was liberalised in the 1920s to just two defenders between the attacking player and the goalline while the inside right (8) was pulled back to cover. This was populised as the W-M and left players numbered:
11 9 7
10 8
6 4
3 5 2
1
In the 1958 World Cup Brazil pushed another half back into the backline, and pushed the 8 back to cover again. Other teams copied, leading to 4-2-4 and depending on whether you pushed back your left-half or your right-half 6 or 4 became another centre-half:
11 10 9 7
8 4
3 6 5 2
1
In 1966 Alf Ramsey's side were known as the 'wingless wonders' as he pushed them back to play a more active part in all parts of the game. I don't remember what happened to Ramsey's team, but this style of play became the standard pattern of play in England until at least the late 80s:
10 9
11 8 4 7
3 6 5 2
1
By the time the game became more tactically varied in this country, with 3-5-2, 4-5-1, 4-2-3-1, 3-3-3-1 and so on, squad numbering was common place.