Yep a tiny phillips/JIS screw holds the knob on. To reset the odo back to 0 you need to disassemble the whole unit. its been ages since i have done one but from memory the whole set of drums on the shaft needs removing off the metal frame, there was a metal tab bent over holding one end of the odo shaft in i reckon and then each drum needs to be rotated by hand to zero, but they are all locked together in a sequence and i had to un engage them before i could rotate them to zero.
I am curious, is your speedo face black or brown? i have been seeiing a few 250 B's with black faces lately so i wonder if they are early models where they used up left over speedos with black faces from the 76 TS250 & 400 first or at least the black faces before going to brown?
Can you reset the trip meter forwards and backwards? I don't think i can do that on the TS speedo and that was a PE only feature for enduro use.