Don't know your Canon, but it is almost certainly a copy of a Leica (probably IIIf), and this is the way all screw-mount Leicas load. A half-width leader 100 mm long, tolerance plus a few mm, minus zero, is required, and as you will have found, the loading technique is to pull out the take-up spool, attach the film to this and then insert the spool and cassette held as steadily as possible into the camera together. Then ALWAYS wind on one frame before you re-fit the camera base, and watch the rewind knob to see that it rotates while you make the two blank exposures to start the film.
Some people will claim to be able to load an old Leica without cutting the leader, the only problem is, if you get it wrong, the wind sprocket may abrade small particles of film from the sprocket holes and these particles are hard to clean out! All 35 mm film cassettes were sold with the half-width leader when your camera was current, IIRC this changed around the end of the 1960s when Leicas no longer dominated the market.
Regards,
David