440/450X21 is a common tyre among many mid 1920's cars such as the Sunbeam, Alvis 12/50 and Aston Martin. These cars would have fitted a set of 440/450X21 tyres. Some cars of the early 1920's fitted beaded edge tyres in the size 815X105, and may well have opted to fit 440/450X21 as a more practical option.
When the Model A Ford was first produced it fitted 440X21 tyres. Today instead of manufacturers making both a 440X21 tyre and a 450X21 tyre, they instead tend to make a 440/450X21 tyre to cover both bases. So if you are wanting 440X21 tyres for your Model A Ford or other 1920's car the 440/450x21 Lucas is a very good value option.