You are buying one key only.

The second photograph shows the range of book type money boxes that your key can be made to fit immediately.
There are a number of manufacturers, who have made these money boxes during the 20th Century.
Pearson,Page,Jewsbury, are perhaps the most frequently encountered but Taylor, Law and Company Ltd, H. Hipkiss & Co. Ltd and  a small number of other firms also produced tens of thousands of boxes.
The manufacturer of your box will be found in impressed letters on one end of the money box,

The full range includes the Post Office Range, ie all colours carrying the Post Office crest or Coat of Arms or commemorating Royal Jubilees or Coronations etc., the Westminster Bank Book, the District Bank Book, the National Provincial Bank Book, Lloyds Bank Books, Yorkshire Sub Districts Electrical Savings Books, Tall Acorns from little Acorns Book the Manchester and Salford Building Society Book and many, many more.

😊😊❀️😊😊Please stipulate, if you have not already done so, by independant email message, which manufacturer made your box/es and which financial istitution issued your box.

If your Saving Book is not shown in the photograph which accompanies this list enquire with the key maker who will confirm whether your key can be made and what arrangements need to be put in place.

Unfortunately the key maker does not send keys to addresses outside the United Kingdom.

Please if in doubt, ask!