OR two widely spaced gum breaker ridges near the very top and bottom of the stamp
Painting with a broad brush.
Details of some exceptions was not include by design. So if you are going to add details, do just stop there, add all the details for the differences between the correct base stamp to be overprinted. There are many such as no straight edge and only certain plate numbers were used and, and.....
As to indentation on mint stamps with full gum, there will be no indication in the gum as the gum was applied after the overprint but some pressure was applied to the paper when the overprint was added. No you are not going to see paper dents as found with engraved stamps.
Frankly anyone buying a K-N OP needs a cert to be sure of their purchase, but that is nonsensical due to the price for some of the stamps, less so for used. This is becoming true for almost any overprinted stamp today for example there is a huge number of so-called Darrah Shanghai Overprints regularly coming out of Romania.