Squire Wilson points you to a knowledgeable source, Lianna Spurrier.
But you will learn more quickly if you visit her web site at
https://rectanglecoins.comand view her description of "Silver Set". Look for the Shonai coin.
Finn235 thus refers to the Shonai clan. As the time went by the silver content in these coins dropped and the currency was basically devalued. The Shonai clan added new chop mark(s) to mark the coins they knew had higher silver content. One of the chop marks looks a lot like the validation stamp you see on all ichibu (top center of your first image). The other mark looks like an upside down Y sticking half way out the top of a square with a nearly total opening on its bottom. Neither mark looks like yours.