On Battery-Maker's images, I see 1931 on the top one and 1929 on the other two.
My bad, I should have been more specific....
The dates you refer to are Copyright dates.... I have a 1934 battery with the same 1929 date... But the date of manufacture
is molded into the sealing wax. When they went to the three pin sockets, they used a rubber stamp of the date of manufacture.