[tgasson]

It's 4. The columns are the digits 1, 2, 3, 4, 5, 6, 7, 8, 9, 0 And the rows are the character used for that digit at that point in time.

At some point later 5 mutated from looking like 4 to as it is now.

[acqq]

Also worth comparing with the numerals used today in Arabic:

http://userpages.umbc.edu/~samir1/620_Project_2/L3T1_1.htm