Grandad even has a pic of inside a canon!!!
In reality inside a cabinet at the Thursford collection, sound stage. well worth a visit,( Norfolk UK) if you like to be flattened ( figuratively speaking ) by a Wurlitzer and other steam stuff.
My Canon pelix ( slr no mirror ) had little bugs develop inside and would walk around on the prism, apparently eggs in the lens glue... Canon fumigated it !