Hi
SBM,My

on a good score there!
What you have there is a good example of an early Series 2 twin rail still in its working clothes. The Briggs is a Model 80102, manufactured on the 28th October 1975 which would suggest that the machine chassis was manufactured at the Thebarton factory in S Aust. early to mid 1976.
I also notice that the inner half of the cutter clutch has been replaced at some stage by the later design grub screw type, which is a good thing as they are about $70 to replace...less vibration also.
Plenty of life left in the cylinder reel....this should make an excellent restoration.

By the way, my machine is also a 450520, from around the same time.
