Based on the pre-war German Zenit, imported by Millard Brothers before the war, it was improved by extending the cocking lever strap to make it clip over the body and giving it an adjustable rear sight. Manufactured by Milbro from 1950 to 1977 there were a number of minor changes involving the front and rear sights, it also appears that whilst the early models had an ‘anti bear trap’ device (called in the literature safety trigger action) fitted so once cocked the trigger could not be pulled until the gun was closed it was not fitted to later models.
Most air pistol books have a write up on them.