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This is by no means a definitive guide (I am well-aware that there are makes and models of typewriters out there which do not follow this method), but here's one way of removing the platen from your typewriter, should the rubber require replacement, should the feed-rollers require attention, or should you need to remove the platen or rollers for any other reason:
Like my previous posting in this board, it's a link to my blog. Enjoy!