What does a 1872 us trade dollar 420 grains 900 look like?

It wouldn't look like anything because no genuine Trade Dollars were minted in 1872. If you have a coin with that date it's a counterfeit.

There were some silver pattern coins dated 1872 that were struck in 1873, but these are already accounted for. They are versions of the Liberty Seated Dollar.

You can see pictures of genuine Trade Dollars at www.coinfacts.com