Thanks for the replies.
Dimensions: about 68 mm wide, 73 mm tall, 6 mm thick. The nakago ana is about 28 mm high.
Material: I believe it is steel, just based on the appearance of the metal on the inside surface of the nakago ana. I have some background with steel and wrought iron through bladesmithing, but I am not familiar with other forms Japanese iron might take. It does not seem to be wrought iron in any case.
Where the gold remains on the rim you can see cross-hatching. This is what made me think that it was gold applied to the rim, and not just paint or brass. On other parts of the rim, the rusting has completely obliterated the gold and the pattern.
BTW, I am (perhaps obviously ) not familiar with terms and history here... so please feel free to correct my terminology or point me in useful directions. I realize I have jumped into your forum with both feet and appreciate any help given.