Shifting bridge and nut
Instruments- discuss specific features, luthiers, instrument problems & questions
I ordered a set of the tuners on Ali Express for under $15 for a set of 4. I used PayPal to avoid sending a credit card number. They arrived promptly, considering their origin in China.
They ARE 4 to 1 planetary geared units. I wouldn't give them the 1 in 5 rating another buyer gave but more like a 2. They look good but the quality is poor. Two of the four had flash on the castings which should have been removed before plating and needed clearing before the retaining barrels would insert. All four were a bit jerky in turning, and the barrel threads may or may not work to full depth.
Aside from quality, they have a downside in that without making special spacers, they will not fit wood less than 0.4 inches thick, and maybe not under 1/2 inch so they aren't good for side mounting. They are described as "banjo tuners," however.
I haven't tried fitting them to anything. I'll almost certainly put them on a stick before risking any bodied instrument on them.
Research is never free, and $15 isn't a big expense.