MildBill–Welcome to the Forum. I’m sure you will enjoy it. You have ID’ed the ECS headstamp correctly. The ECS 42 & 43 cartridges are standard .45 ACP for the M1911, or any arm made for .45 Automatic. However, if your going to shoot them, they are Corrosive Primed. They are by far the most common round from WW-II still found today. Evensville made so many BILLIONS of.45 ACP in 1942 & 1943 that the military shut the plant down even before the war was ended. They had enough in the warehouses that even if the war went another 10 years, they would not have needed to make any more. You can buy full boxes of 50 of them at almost any gun show today, almost 70 years later.
Are you sure that the 5mm cartridge you mentioned is 5mm. There are a few rifle cartridges of that caliber, but not many. If you post a picture or at least give us the dimensions, we can ID it for you.