I've looked at a Canon P and its cassette chamber is built to the DIN/ISO standard.
The Canon casette that I use says 'Canon V Japan' think this means that it is made for Canon V could be Canon went to DIN for the Canon V and post models like the VI and P that I use.
The above V casette model has only 1mm of free axial movement in my P.
It will fit in a Nikon F2 but won't open.
It won't fit into a Zorki I d # 4925xx ie the baseplate won't close but this is because the lock arm cassette opening device has been modified (bent) for ( to locate)factory DIN/ISO crimped cassettes...