The bike came to me with broken black stem shifters, and the deceased previous owner had obviously ridden it a whole lot of miles like that, given that the smallest sprocket was heavily worn.
I bought the bike from his son, who said his dad used the bike over a very long span of years, riding along the American river trail.

So I found white stem shifters on Ebay, being sold new, but with rusted clamps and cables.
Turns out the clamps were for 1" tubing, and that the white ribbed housings were fine. I got 4 sets for only $15, and used the white levers and wing nuts on a standard 1970's Simplex downtube clamp.
I liked the way that the white plastic further sets off the sort of "unlikely" theme for a bike being used on today's more-sporting recreational rides.
And I think that these levers might have been destined for children's bikes, possibly into the '70's.

The molded-plastic friction surfaces need a few hundred miles of break-in time before the friction feels crisp. Up until then, it felt somehow vaguely "hydraulic", with no feel for when the shift lever cracked loose from it's existing position, and very little grip to resist it then moving too far.
I think that the metal washers eventually burnish some metal into the plastic friction surfaces.

I would have liked to use a chromed clamp, but I just had this one handy. I left the adjusters in place just because they added to the look.
Originally I had lined the shifter clamp with clear tape, but the shifters kept slipping down the downtube so I removed it. No more slipping: