With the serial #4088598, NOMDA 1980 suggests a date of early 1963 for The Blue Bomber
1970 721 – note Page Gauge, paper table and variable release lever…
1970 721 – body casing quick-release levers
To open up the 1963 721, you flip it on its back and first pull off the bottom cover. It’s held on by friction clips.
Then you loosen (not remove) the 4 screws that look like this, and slide open the latches.
Flip it back right side up and lift off the top cover after removing the carriage, platen tray and feed rollers, and flipping up the margin set levers.
IBM Typewriter Colors, 1961-1967 (Selectric production began 1961, Model C production ended 1967), provided by ProfessorC