I have a 7-speed indexed twist-grip gear-shifter, but it was set up incorrectly: it has eight positions, the fourth is where the numbers start showing, and while it hits the top and bottom gears, it has an 'empty' click that takes it from top gear to top gear and it skips from ring 3 to ring 5 with just one click.
I've read the response to How can I adjust index shifter so it doesn't skip the second chainring? but this is a 7 speed shifter (masquerading as an 8 speed and a 6 speed), not a 3 speed, so I don't believe the answer there applies here.
It seems that the twister is giving too much cable for each click (like a 6 speed shifter) and has its own position limiters which allow extra travel (like an 8 speed shifter). I've fiddled with the top and bottom limit screws on the derailleur, and with the [barrel] that simultaneously changes where ALL the indexes locate, but while I can get it somewhat aligned for all sprockets, it still does the 'not quite engaging properly' thing for a few of them, and I still have extra clicks at top and bottom.
How do I reduce the amount of cable each click gives? There's an intriguingly yellow 'c-deck plus' sticker on the twist-grip that looks like it's hiding a screw, and a couple of tiny Allen key looking things that I'm guessing are holding the unit together.
Update: the shifter declares itself to be a 7-speed shifter on the front, and the part number bit towards the back reads 'Shimano SL-RS43-7 Singapore 22.2' which mentions '7' so... presumably it's a 7-speed.
I don't know where to find the serial number of the rear cogset..?