He owned a .38 Special, yet they found a .380 cartridge at the scene. Odd.
Shell casing,not cartridge.
I think the 380 is a nomenclature problem. The term “380 MK1 Webley” refers to ammunition for the Webley Mk I revolver, which uses the .38 S&W (also known as .38/200) cartridge. This cartridge is distinct from the .380 ACP (Automatic Colt Pistol) cartridge, and the two are not interchangeable. The .38 S&W (or .38/200) round was designed to be used in Webley and Enfield No. 2 revolvers.
But Oswald used a 2” Victory Smith & Wesson. And it was chambered in .38 S&W (not .38 Special) It fired a 200 grain bullet and is a shorter case than the 38 special.
A cop back then would likely have noted it as a “380” (in the British Webley short case sense)
Here’s a photo of it. https://tjsmilitaria.co.uk/products/380-inert-british-1940-dated-webley-revolver-brass-cartridges-rl-royal-laboratory-woolwich?srsltid=AfmBOoqPkkY5eBLdqCdSXkagUVWXWFdYAYQ19fVkWxK53REcPA1VP96o
I can't remember where I recently read it, but an eye witness stated it wasn't Oswald who shot officer Tippit.