A Rare Box Of Unused 1940s Slazenger Tennis Ballls
I presume the date on the balls is when they were made, 
The box states that they were the official ball for 1939
UK BIDDERS - 2nd Class Recorded
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On 06-Apr-24 at 09:08:34 BST, seller added the following information:

Just to say, these balls are not suitable for use as the rubber has hardened over the years of storage,

They are purely collectors items.