Malcolm, all 1969-1977 cars have cam adjusters on all four wheels.
1978 cars have only on the front torsion bars, 1979 onwards four fixed mountings :-(
"Early" and "late" are very relative meanings as the first Haynes manual was published in 1972, the last in 1986... :-)
The method you describe with the centrally positioned jack is an old 2CV trick, it is particularly useful on them when you start assembling the suspension from scratch. (and don't forget that with their interconnected suspension is easy to end up with two diagonally placed wheels holding the car level) .
Renault officially recommended the use of some special scales to check the spring loading at each wheel (a "scientific" view of the method you describe!). I have found it unnecessary because there is no interconnection on our suspension, and because the workshop manual gives a (rather accurate) baseline for the initial torsion bar setting.
kavinandkate, both fixed and cam type front torsion bar mountings are held by two large and one small bolt. Look at the top bolt, if it appears to pass through a fixed hole, you have the latest fixed mounting. If it passes through an elongated oval hole, (and there is a series of other holes below), you have the cam type mounting.