Ooooo those are rare and very hard to find. Honda never made those available in their parts catalog. It just so happens I have one I could sell you! haha and I'm sure everyone else here probably has a spare one laying around too.
According to the service manual update the window was intended to provide easy access to the clutch cable to make it easier to remove & install. The rubber cover, or something suitable, probably should be in place to keep dirt & debris out. I Googled the part number and came up with this site: http://www.hondarestoration.com/nosResu ... ourer-1959 . They seem to be in stock.