My current earliest IBM Model M based keyboard, and one of the earliest Model Ms ever recorded! Whilst the overall assembly design 'language' is clearly Model M, there are some unique features to such early IBM Wheelwriter 5 (and Wheelwriter 3 and Quietwriter 7) keyboards including O-rings around each populated buckling spring barrel and 'cushionless' flippers.
See my website for more info on Model M family keyboards and buckling springs!
