Pump sizes are usually round inches, measured when slightly compressed for the mounts.
If you're looking for a specific make / type then you need to provide illustrations. Royal Enfield are likely to have supplied trade-marked inflators from about 1930 through to the mid 1960. There will be a number of styles and not all correct for a WD machine.
Yes, I have them listed as 12,5" length for Royal Enfield, I have this picture, but that's from a 1940 Model C, not sure if that would have stayed the same for CO?
With all the different makes and models of WD motorcycles and the vast array of different pumps and their lengths, I'm just happy to find any period pump that fits. It's an absolute bonus when you come across a correct one that's listed or shown in a parts book. Not that, that is always cast in stone of course! Ron