There should be approx 12 small round headed slotted screws. The one I am working on only has 5!!, the rest are missing. Ive tried using a 5/32 BSW screw and its too small, and I've tried a 3/16 BSW screw that seems to be big, I don't want to force the screws and strip the thread.
I assume with the age of the machine being mid 1950's, that BSW screws were used?, there was a thread called BA, but not sure if they were used on this type of escalator.
I know one of the screws would have been countersunk, that one I have.