I bought one of the "gizmos" from Model Expo back in the eighties.
Simple enough device--my only gripe was that it was a bit short for serving an entire shroud.
Ir was constructed of a base of a bit of poplar (or some similar wood) about 2 x 4 x 30 cm. Then, a couple of uprights, 1 x 2 x 8 cm. The uprights are centered on the width of the base, which allows space to clamp the base to the bench.
Between the uprights is a aluminum tube with a pair of matching press-fit plastic gears. The tube was set into press-fit bearings (nylon bushings, if memory serves) in the uprigths.
Just above that axle, using identical gears (they do not have to be--they just have to be the smae at both ends, and mesh together). One of the gears has a short hollow tub with a crank attached. The other end has a matching tube, which has an alligator clip slipped over it, Since the alligator clip is on a slip-on fitting, you can change which end is the fixed end--but, I fould that opposite the crank is best for me.
For a shroud, you need a length of line which will go around the deadeye, up and over the mast and back around to reach the next deadeye along. Using a bit of butcher's or kitchen string makes for a handy 'try' gauge
What worked for me was to fit a bit of the shroud in the clip, thread it through, then hang a weight on the other end--like a big 2" bulldog clip. Then, clove hitch the serving thread up near the clip, then, start cranking until the amount of serving is achieved. (Remember that the leading shroud on a mast was generally served its entire length to protect it from sail wear--so that serving will go up, over the seat, around the mast, then down whatever length is to scale.) Finish the serving with another clove hitch. A dab of adhesive is your friend. Then, move the line down to where it needs serving around the nest deadeye, and do that up. A person fully dedicated to accuracy (or working in scales larger that 1/48) will want to rotat the gizmo the opposite direction.
I found that holding the spool of serving thread so it passed over a bit of beeswax as I cranked was all that was needed to get it to grip and hold the shroud.