Old football programmes often had a centre page spread of the teams in formation with jersey numbers.
I'm a few thousand miles from my programme collection but here is the first one I could find online, which illustrates the numbering system (basically each tier of players is numbered sequentially from right to left starting from the goal keeper:
When Herbert Chapman moved the centre-half (number five) to sit between the two fullbacks, thus creating the WM formation the numbering convention became less logical. One would need to look at the Huddersfield and Arsenal teams of the 1920s to see if they renumbered sequentially or if the three fullbacks wore 2, 5, 3.
(It always annoys me when people (players, managers, pundits as well as fans) refer to central defenders as centre halves - centre half is centre midfield!)
And while I know Bobby Moore adorned #6 jersey in a back four, it seems strange to me that convention might take the 6 into the back line and leave 4 in the midfield. Did centre backs at other clubs wear 6 in a back four defence?
As for the forward positions, 8 and 10 were the inside forwards, and 9 was the centre forward. 7 and 11 were always the wide players, whether being outside forwards, wingers, or retracted as wide midfielders. Thus when two forwards were used it was the number 8 that was retracted to midfield and the number 10 that played alongside the centre forward, number 9.
By using the principle of sliding the positions back and forth in the diagram, (as with the wingers 7 & 11, centre half/back 5, and the number 10), a back four's central defenders should be 2 and 3 as they are the equivalent of the old fullbacks, and the wide defenders who often push on as wing-backs or more accurately wing half-backs (full back is another misnomer if they aren't staying back) should remain the same number as the old wing-half-backs, 4 and 6 - thus a back four would be numberered right to left: 4, 2, 3, 6.
