[Nikbul]

If you look closely, 1 in 1971 is a lot closer than 7 that written in 109. Also as mentioned below, defining stroke in the middle with 7, and how it is written. For number 1, writing starts with upper left and goes down, that makes sense for translation from 1 to 0. For 7 you do same but middle stroke takes your hand off paper so there is no direct and smooth translation to next number. That's my reasoning for interpreting it as 109, rather than 709.