When I put “Muromachi Era” for the “Era” in the explanation of the “Hozon Tosogu Tsuba Mumei (Kachushi)” shown on our website, a customer pointed out that “If it’s Muromachi Era, it should say ‘Ko-Kachushi’ on the NBTHK certificate”, and so I checked with an NBTHK screening committee member.
It seems that not all pre-Edo Era works are sure to have “Ko-” attached to them, but (in principle) it seems that they apply “Ko-Kachushi” for Murumachi Era works and “Kachushi” for Edo Era works.
Incidentally, it seems that the default assumption is that “Owari” and “Kyozukashi” are Muromachi Era. When a work is through thought to be from the Edo Era, they assign “Owari (Edo Era)” and “Kyozukashi (Edo Era)”, respectively.
This was a learning experience. I’m grateful that it was pointed out to me.