I have a 4x5 Crown Graphic with a side mount Kalart synchronized rangefinder. It appears to have a part missing. The top of the rangefinder has a hole with three notches. Looking down into that hole I am able to focus. It is impossible however to do this with the Graphlite mounted. Photographs I am seeing show an angled piece fitted on the top of the rangefinder. Suggestions on what I am missing and where I might find a replacement?

look at the last page of the kalart manual https://graflex.org/speed-graphic/kalart-manual.html

There is a plate that attaches with a screw to cover the hole when a Focuspot is not installed. Make a plate from .015-.020 steel, aluminum, brass painted flat black.