How come the 1 in 1961 on block 20 looks like it’s written in?
Exactly.
there are now two things wrong with line 20
the font of the “A” on line 13 appears different
from the “A” in the date on line 20.
This could be one of those rotating stampy thingies where you can change the year by moving to the next number. the "1" would be 'fresher' compared to the '196', due to having been put into effect at the beginning of the year.