It is a stacking lunch pail as Mart stated earlier. Since the earlier post some time ago I have come across quite a few in antique/vintage stores and have found they go by different names.
Modern day they are called tiffen boxes - Tiffen being a an American term for a little bite, but it is more common for Indo-Europeon origins. You can also find it under Bento Box Carriers.
This style was used widely by miners, so we think of them when we see this style. However, this was used by many workers in the late 30's onward.
Search vintage stacking lunch box, tiffen box, tiffen carrier, Bento carrier stacking....and you will see more than you ever dreamed of.