Mity2,as 03man has said,1/4'' diameter.My 6th series Nagoya has a 1/4'' diameter cleaning
rod also. Can you post a picture of the left side of the receiver where the serial numbers
and the series mark are,since you don`t sound like you`re sure of the series type?
oh I'm sure about the series. I was just not too sure about Nagoya.
But i just found this website (radix.net/~bbrown/japanese_markings.html), and i guess its Nagoya for sure
In older board, i was given as to which dimention my gun's cleaning rod should be, but i donno how to get old thread.