I don't know if there is any specific confirmations of an over date, but if you look at the top left corner of the '9' it looks at if there is a bit of a trace of another number. It looks like it could be a '7', perhaps one of the workers started repunching the dies and accidentally made the date to condensed, or maybe it wasn't centered properly and they had to either polish the date and start over, or repunched over their mistake and then polish out the remaining details of their mess up but accidentally left a bit behind.
This is just a guess that seems like likely to me, given that there is a '7' in the date.