As I noted in your other thread, it seems like this is an overstrike, and the earlier coin's obverse became this coin's reverse, and the other way around. A really cool feature on an already great coin!
I know nothing about
medieval coins but this one sent me looking at other examples pictured online and I saw several with similar shadows of the design of the opposite
side as mentioned here. This is exactly what I would expect not from an
overstrike but from a die clash.
Clashed dies are the result of the two dies being struck together without a
flan in place between them causing damage to the weaker of the two dies. On ancients, this usually shows as damage to only one, the
reverse, die but it seems possible that two very simple dies made of the same metal might damage each other allowing both designs to show in
incuse on the wrong
side.
While the examples are ancient, my page on die clashes might explain the theory better:
https://www.forumancientcoins.com/dougsmith/brock.htmlObviously it would
help greatly if someone has another coin of the same die set with the shadow marks in the same places. All coins struck from
clashed dies after the clash will show the damage in the same place while a second strike would be different on each so the
undertype evidence could be in a different place. I am also wondering if a very thin
flan might allow a
bit of a shadow from the lack of pressure opposite the more deeply cut parts of the die. This would not require damage to the die from an accident but would show on all coins with very thin flans. Does anyone expert in
medieval silver see possibility in either of these ideas given what you know of the minting techniques used for these coins?