Good tip. I have found information on caster (search "Wheel Alignment Explained"), the side angle of the steering pivot axis, and apparently this is what the backward/forward setting of the endcap affects.
"If the caster is different from side to side, the vehicle will pull to the side with the less positive caster" and this is exactly what is happening. The LHS shock absorber endcap is further forward, which means less positive caster, and the car pulls to the left.
"Like camber, on many front-wheel-drive vehicles, caster is not adjustable.", however not so for the Fiat Panda. The new Fiat Panda Haynes manual for post 1995 models has recently arrived and I will check this before making an adjustment.