Hi Robert and all,
I've seen a few different "versions" of m116s that show up with blank backs -
1.) Black and White Proofs - only a couple of these are known
2.) Large number of Proofs with no captions, black backs and obviously handcut off a sheet. Paper stock appears to be the same as normal cards.
3.) Ad pieces on thicker stock that has been cut up.
Does the paper stock look thicker at all to you? If you look at the 3rd scan I have posted below, the quality of the paper stock is both thicker and maybe a little "rougher" as these came off of advertising panels. Judging by your scans, I'd say your cards fall into the 3rd category since they both have color and appear to have some jagged cuts.
Here are some scans of cards I have owned/still own from all 3 categories for you to compare yours to....
Item_24872_1.jpg
m116ads.jpg
m116adsback.jpg
m116mitchell-001.jpg
m116mitchellbk.jpg
panel.jpg