I have a low-ish serial number model 306 Harrier 41 autodrive and that I believe to be 1989.
Mine has the finger trigger height adjuster not a screw type as on the machine in photo. It does though have the same engine. No OPC, which I believe was reqd by 1995.
I also have a Harrier 48 from 1987, which has the height adjuster similar in operation to the type on the machine in photo and has lost its adjusting knob, the same, leaving just a hexagon.. The rear flap is of the same style as my 1987 48cm mower, with the triangular ends overhanging the chassis side.
So I will have a stab at 1987/88..unless, as suggested, a model has been assembled at another time to suit a “Shed” price, but still pre 1995.
C.