I am always tuned to anachronisms. I spot the actor pulling out the crown to set his pocket watch in a civil war play, or hear the clock chime in a performance of Hamlet. Historical accuracy isn't high in many films/plays. Highlander was an exception in that most of the dates, battles, and people were fairly accurate. I missed this error about the era of Masamune ( pun intended).
FWIW, the katana appeared in its earliest and simplest form in the 7th century AD. It was improved in the 11th century, , and made into the current form by the 13th century. It has had few changes since then.