A little bit more information on that last cover scanned. The return address is the General American Car Company, Chicago. Further research confirms that it was a subsidiary of the General American Tank Car Company, a manufacturer of railroad freight cars:
The General American Car Company was [a] subsidiary of General American Tank Car Corporation. Known to have received orders in 1921 for 500 refrig. cars for GNR; 600 rebuilt hoppers for IC; 600 freight cars and 14,000 tons steel rails for Chinese government. In 1922 for 700 40-ton refrig. cars for SP; 1,000 50-ton single sheathed auto cars and 250 40-ton stock cars for NP.
As for the addressee, Mitsubishi Goshi Kaisha, although part of the well known Mitsubishi family, it was a subsidiary corporation dealing with the export of steel and iron and machinery:
The connection between the two firms is certainly understandable, but alas, not connected at all with the automobile industry.