I did a post years ago about some really cool printer's scrap cards where the backs were covered in test prints from various sets.
Here is another one that is on the same level as those ones were when it comes to color and amount of different test prints on the reverse.
This John McGraw card was obviously hand cut by someone during the period in which T206s were being produced. These cards come from sheets that were used to test the printers and sometimes they would send the same sheet through multiple times.
When they did this, they created these sheets that have lots of messy backs and it appears that some folks didn't care about the back and cut out the cards to keep anyway. These cards have gained in popularity over the years and this example sold for $5,036. Obviously the multiple prints on the reverse is the reason for the high hammer price.
The prints on the back are of Bradley and Manning as well as tests of two different advertising backs, Tolstoi and Piedmont.
Here are some images of printer's scrap that I posted about before just for comparison.