Hi Grant:
The Polsten and Oerlikon were similar and used the same ammo. They were popular with the Army and Navy. The Airforce tended to prefer the Hispano-Suiza which used different casings (larger base) and it looked radically different from the Polsten. This is, of course, a generalisation.... I'm sure that as soon as this is posted someone will come up with a picture of Polstens mounted on aircraft!
The actual story of the development of the 20mm in Canadian service is well told in Clive's book, "Tools of the Trade". There is also lots of detail in the Army Historical paper number 23 (if I got the number right) which is on line under DHH in the DND pages.
The Polsten miniature model is already available its simply a matter of convincing the manufacturer to sell the guns without the added expense of the carriage.
Cheers! Mike