Dumb question if I may. I have an old 5-speed block (Shimano SIS), an old Shimano 105 5/6 speed rear mech, and Sachs Huret Rival Aris 6 downtube levers. I always thought that the indexing was driven by the levers. I.e. the mech itself is free moving rather than jumping a certain distance each change, and that it is the single click in the lever that moves the mech a defined distance and thus changes the gear precisely.
Is that correct?
If it is, then I can pretty much change the mech with anything suitable for the thicker chain width of a 5 or 6 speed block so long as the levers still work,
Is that correct?
Or am I being a total bike-numpty.....
You are half correct, in that nearly all if not all systems have the indexing click implemented at the shifter.
However, that is not to say you can pretty much use any mech. The simple reason is each click at the shifter pulls a certain set amount of cable, while the rear mech (or for that matter a front mech for an indexed front system) has to translate that precise amount of cable pull to traversing the distance of exactly one sprocket up or down. Obviously that translation is determined by the design and geometries of the mech. So for instance all Shimano SIS rear mechs, which include nearly all Shimano rear mechs made since the mid 80's, are interchangeable in terms of cable pull translation standard (i.e. for the same cable pull they all traverse along the sprockets by exactly the same distance irrespective of its vintage or "speed"), so can index correctly, within reason regarding chain/pulley width tolerance etc., with all Shimano rear shifters made during the same period. This "standard" was only recently abandoned by Shimano when they implemented their DynaSys 10 speed system for their mtb equipment.
So going back to your suggestion, according to
Velobase it appears your Sachs Huret Rival Aris 6 shifter is not Shimano SIS compatible, i.e. the amount of cable pulled each click is not the same as e.g. what a Shimano 6 (or 5, since it would be the same) speed shifter would deliver.
That however is not necessarily the end of the road for that shifter. It should not be too difficult to find out exactly how much cable your Sachs Huret shifter pulls per click, and compare that with those in the tables
here, and see if it is close to any. Even if it isn't, and if you are interested and have the inclination, there is also a small chance that you might be able to work out, in combination with the mechs' ratios, if it will work with certain mech to offer just the right traverse for a certain rear cluster. Or perhaps it can be set or converted to a friction shift.
Hope it helps.
I replaced both shifter cables and adjusted the derailleurs last weekend for the 1st time ever, and it is very satisfying to know you no longer depend on others to do it, isn't it? The gear shifting on that bike has been smooth all week. I don't know how much cable stretch the shifter cables have, but it's possible we'll both have to do a follow-up adjustment in a few weeks?
I almost ruined the FD cable (again), but managed to avoid having to put another one on. I guess I shouldn't expect it to go perfectly, given that this is only the 2nd time I've adjusted the derailleurs, as only about 4 weeks ago I replaced the FD and RD cables and did my first ever FD and RD adjustment. Anyway, there's no more chain rubbing, and the shifting is as smooth as I can get it for front and rear 
