I have a Retina IIIc. The top cover is easy to take off. Pull up the rewind knob, you shoud see 2 small screws. To remove these, you will need unsrcew the rewind knob by locking something into the fork inside the film chamber.
There is a third screw for the top on the end by the meter. The top should slide up with some gentle rocking motion. Watch for various springs etc. as you gently lift it up.
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