The Imperial was introduced for the 1968-69 season. At this time there were no two-stroke versions.
The Imperial - like yours - was introduced for the 1972-73 season (the 1973 Range). Victa would introduce their special version of the Series 70 Mark 2 engine to the Imperials. This engine, fitted with the Mark 2 decompressor, also featured a lower compression (via a head spacer), and a special version of the G3 carby (the 3-357), that removed the air vane governor.
I'm not sure of the end date: possibly as early as 1976.
thank's jack,today i got a catcher,problem is i had to buy two mower's to get it,another imperial 16 and a old rover,got both running,victa is backfiring but working,rover motor purrs but has me wondering,the clutch set up is off and is missing something,will post pics soon as i work out how,cheers