Hi Mark,
Yep, that's indeed a 1978 Victa Power Plus Self Propelled (WV-173-A8) using the MKII Modified Auto Drive Gearbox & Drive System, A G4 Carburettor, Series 70 MKIV High-Compression Engine, and mechanical (Points Based) ignition.
The muffler in the pictures seems to be the incorrect one as far as I can tell, the Power Plus (HC) system used a muffler that had been tuned to the higher compression engine, this muffler is identified by a hole punched in the exhaust outlet.
While the linkages for the clutch lever are simpler on this gearbox as compared to the earlier VC gearbox Blumbly has posted, you definitely are missing a few Items, in particular I notice there is no tensioning pulley.
I will have a poke around and see if I can get you a few photo's of the set-up as it should be to help you identify all missing parts, I may also be able to help you source some of these parts.
I can't wait to see what you make of this one mate, it looks like a good candidate for a restore!