So here is an IBM Selectric on half of its clam shell. To get to the guts, there was some prying required at the back and then two leavers in front, one on each side. The platten or rubber roller around which the paper winds when typing, it came off first. There are two tabs, one on each end and it comes up easily.
There was a big hum but this went away when the cowl was replaced and a the body was given a few "love taps."
There is an insignificant problem with the margin release as when Return is hit, it does not go all the way to the left. This may be remedied by clearing all the margins and starting afresh.
fifty-year-old machine is the way my fingers knew what to do, how to remove the platen