However, in medieval times, “bulletproof” meant exactly that… the armorer would shoot a musket ball at the armor he had just built, and the dent was the proof that it had been “bullet proofed”.

I didn’t read all of the responses on this aspect of the episode, so please forgive me if this has already been stated, but I never saw mention of one crutial component to creating a mark, in both the responses that I read, or on the show.
Blood.
A bruise (mark), is created due to blood coagulating outside of broken vessels, under the skin
Unless you have blood flowing through your test subject, you cannot test for bruising.
You can test for a scar, as that is caused (initially) by a tear in the skin.
Thus, if the ring made such a deep impression upon impact, as to actually cut through the skin, then it can be assumed that a scar would result and that scar may be similar in shape to whatever it was that caused the tear in the first place, as long as the tear was in the shape of the object that impacted the person.
If the tear is misshapen, compared to the shape of the object that struck the person, then the resulting scar will not have a similar shape to the object that struck the person.