To me, the UA and the Marx 21 have some significant differences even in the shells. This seems especially so in the very forward nose section where the UA loco's have a distinct "housing" for the headlight, you can see the "bump" where the bulb is inserted and the number boards are printed to appear that they protrude from the side. Whereas the Marx 21 hood is just flat with a hole below the front edge for the bulb and the numbers are lithographed as if they were flat. At least that is the way they look in the pictures, I have rarely even seen the UA loco's IRL so I am just speculating through the pictures.
I don't think that UA ever made a specific set to go with these. On the few (rare) occasions that I have seen them they have always been separate items and the only box I have seen them in was just a two item cardboard box designed to only hold the loco's.
In the book "Marx Trains, Vol II" by Matzke and published by Greenberg, the Chapter on Seven Inch Cars by Larry Jensen talks about the relationship between Marx and Sammy Bergman who owned UA. In his view, the emergence of the UA gauge O trains in1949 was a complete surprise to Marx and very unexpected given their friendship. This was especially so in the RI diesels as they competed directly with the then newly introduced Marx 21 (candidly I think the UA locos are much nicer). This emerging rivalry appeared to spur the initiation of work on creating the seven inch cars in January 1949 as direct competitive response to the UA trains. In the end, Marx won and bought out UA and it was rumored sent the tooling for the trains to Mexico.
They are beautiful.
Don McErlean
