It appears to be built on a S1 1600 chassis, but with a 1300HF engine.
Not what it appears...... Chassis number is +1 from the Series but the car was shown at the Geneva Motor Show in March 1969. The engine is a 1600 (block height) but seemingly an early Var1016 based on 342 crankcase (my info is about 5-10 of these were built in 1968) prototyping the 818540 - has standard DDHF42s, and a modern alternator but still mech fan. The front subframe is unique as it lowers the engine and does away with the transverse leaf spring in favour of coil over dampers. The satin black and pop rivet scoop and flares are ugly and later additions.
Reviving an old thread I just wondered if there are any pictures of the suspension of the car - I'm sure I've seen some somewhere but can't find them now. Doesn't it have independent rear suspension too? Not that I'm thinking of modifying my car but always intrigued as to what has been done!
