I'm replacing the rear shifter on my old Kona mountain bike from the late '90's. It has an 8-speed rear cassette with a Deore XT derailleur. I got a basic ST-EF500 8-speed Shimano shifter/brake combo, but I was having a bear of a time indexing the gears. After about 10 minutes of messing with it, I realized that on the large cog, the display showed gear 2.
So, I went back to the small cog and counted the stops on the way up to the large cog. Sure enough, it was 6 steps on the shifter to travel 7 cogs from the bottom to the top (and the shifter wouldn't go further, which is probably a good thing since I don't want to shift into the spokes). I disconnected the shifter to make sure that it actually had 8 distinct stops and wasn't defective and it functioned as advertised.
It appears, then, that a distance of 6 stops on the shifter matches nearly perfectly the distance of 7 cogs from top to bottom of the cassette even though they are both Shimano. Is this a known thing? Were there special narrow cassettes for mountain bikes in the '90's that I just didn't know I had?