Hi RB.
The short answer is that the 3" Jeffery has less case body taper and a thicker rim, thus would not chamber in the .450/.400 3 1/4" rifles. Likely part of the reason Jeffery did this was so that his new Full Nitro (Cordite) cartridge could not be fired in the older Black Powder Express rifles. (Plus only proprietary Jeffery ammo could be chambered and fired in Jeffery rifles?)
Easy to see when one looks at cartridge dimension prints.

You can reload the 3 1/4" with a Jeffery die set, however it would be essentially a neck size only operation. As well, depending upon the die maker, the 1/4" longer case could have its mouth boogered up if the decapping stem design gets in the way when running the case too far up into the sizing die. <<< Don't ask me how I know this

- Mike