We have about 2 dozen of these WWII cans that we display on our halftrack (along with 10 .50 cal cans). I am by no means an expert on .30 cal. cans, but I have some pictures attached of our cans and some observations below.
So all of our cans have the long latch with the slot. None have a tab that sticks through the slot, so I would definitely say that was added. I have multiple boxes produced by Reeves, Canco, Crown and one that is unmarked. The main difference between the manufacturers lies in the hinge construction and how it is attached to the can (rivets vs. spot welding). Crown produced cans are also a little simpler in the stamping.
Also there are three types of the metal .30 cal cans produced during WWII (as far as I know); the T4 Ammunition Chest, M1 Ammunition Box and the M1A1 Ammunition Box. The T4 was the first to be developed and was designed to be used in tanks and other armored vehicles, usually these are painted in white or red-oxide primer. This box had two latches on either side that rotate to lock the lid on. After some redesigning, we have the M1 Ammunition Box which was the standard used during the war (the majority of my cans and your can pictured). Further refinements led to the M1A1, the big difference being the latch on the side for use with the M1917A1 cradle as well as a new hinge, lid and latch design. These were produced late in the war (around March 1945 if I recall). I have a few photos attached of the three cans side-by-side, I can of course take more/better pictures if you need.
Hope this gives you a little help! As I mentioned, I am no expert just sharing the little bit I know. If anyone has any better info, please feel free to add and/or correct me.
Ty