I used 3 IIIF , 1 IIIC and they were the most excellent viewfinders and fastest focusing and damn sharp. Never vibrates and when you hold one , it stays definetely horizontal. Years later I bought a Leica Minizoom and its viewfinder is 40 years more modern but terrible. I dont know how but you will see that viewfinder will increase your photographs quality. Beware from russian ones , its very hard to focus.
Dont buy a Leica without touching and checking it. Or buy from somewhere with background.