mariachi

Is there such a thing? I have a 5 speed, currently with a stem mounted shifter. The clamp is too small to fit the downtube.
Because it has brazeons for the chainring protector, I don't think I'll swap the cranks and "upgrade" it to a 10spd, hence this search for a single boss downtube clamp. (or a way of making a double boss clamp not look "unfinished")

Airburst

There should be, plenty of bikes only came with a single DT shifter back in the day.

Edit: As for finding one, a co-op might be the best place to look, they're more likely to have a load of old stuff knocking around than a conventional shop.

F red

Yes, common for French bikes.

There are 2 kinds, one is a simple clamp like a normal one, except with only one boss, for frames with a clamp stop.

The other kind has a hole in the clamp on the other side to be fitted around a braze-on boss. Many of the frames these were on started as frames that only needed one shifter for the back, but other builds of the same frame could accept a front shifter for multiple chainrings.

hueyhoolihan

i've used a two boss clamp-on shifter and attached one of the "Problem Solver" shifter covers to one side. they are intended to, as you say, negate the "unfinished" look of bosses sans lever. IMO, a dummy lever looks just as good. they are intended (not the Problem Solver's care mind you as long as you buy them!) made for single speed conversion road bikes.

you can also buy a single non-indexed friction shifter that mounts on the handlebar or the end of the handlebar. some are made for MTB bars and some for Road bars...

my first new bike, a mid 60's Schwinn Collegiate, was a five speed with the friction shifter mounted on the top tube, IIRC. not that that is of any consequence.

onespeedbiker

They are quite common, but you will probably have to buy one with a shifter attached also. Huret made a good number of these, but most have the French boss that are round at the end without the cut out flats or keyed end. Believe it or not, a good number of frames were actually made with only one boss on the right, so Huret made these https://www.ebay.com/itm/HURET-31-8mm...item1e79c53294 that have a cutout for an existing boss on the right. Still if your downtube is 31.8 this should work for your purpose as it has a standard boss.