Any ammo can experts?

[sapper740]

Just bought 10 U.S. .50 cal. ammo cans. The seller only wanted $5.00 for the lot so I figured what the heck and bought them. I've always wondered if there is a way to identify the period that ammo cans were manufactured. They all look identical, right? Well, I was looking at my new cans with an eye to repainting them to spruce them up a little when I noticed that some had a groove in the locking flap (for want of a better term) with a ring holding a cotter pin while others had no groove, ring, or pin. Is this indicative of something or just differences in manufacturing? Any help, as usual, greatly appreciated.