That is a Zenith model 28 (has the idle mixture adjust screw high on the back of the carb) or 228 (has the screw on the left side near the throttle plate, can't see either in this picture).
The adjustable high speed jet was an option for use higher than 6000 feet above sea level.
The really neat thing about it was that you left the original main jet in place, as shown in the drawing below from Zenith's overhaul manual for 28/228 carbs. That way you can revert to the fixed main jet by backing the adjustment of the adjustable jet all the way CCW.
I have one on my Zenith 228, but I've never been higher than about 2100 ft above sea level.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
