What are the other differences if any?
Age.
I don't find the Model 12 Featherweight's recoil to be bad at all. Even shooting one side-by-side with a regular Model 12 doesn't feel much different.
I was actually out with my 'featherweight' today with some 1-1/4 oz pheasant loads -- though, mine comes in the form of a Model 25, the original "featherweight" before Winchester modified it for 'tool-free' barrel and magazine removal and rebadged it as the Model 12 FW.
As far as I know, the basic Model 12 design wasn't changed in any way that would change the ability to fire as soon as the action was closed. Every Model 12, Model 25, or Model 12 FW that I have ever handled (from 1924 to early '60s) had been capable of firing as soon as the bolt locks; and had the inertial lock that requires recoil or a forward push of the action slide to unlock the bolt.
While I can't give you specifics on any particular part, I can tell you that nearly every piece of the Model 12 and its derivatives slowly evolved over the years (many of the parts are interchangeable, of course with the explicit exception of anything related to barrels, magazines, and take-down).
Trigger parts had minor tweaks. Hammer struts changed. Hammers changed. Springs changed. Extractors morphed. Loading gates and carriers had several revisions (some solely to simplify and cut costs, it would seem). Action slides changed many times (though usually only minor tweaks). Action slide lock levers changed a bit. Ball bearings used for the safety detent changed size. Bolts had minor changes. And even the shank size of the trigger group screws changed a few times.
But, overall, the basic design carried on until 1964.