As you probably know indexed shifters have a ratchet mechanism so they click into each gear (in theory).
Friction shifters are your basic units that don't click into place - you move the lever and the derailleur moves accordingly.
I have two bikes with down tube friction levers and gear changes can be a black art until you get used to them.
I like them because they're of that period (as with everything else on the bikes).
The idiosyncrasies are what will give your bike character with indexed downtube shifters.