What are these used for

[Bus]

These beam scales (often called cotton scales) had two different weights or "peas". Used the lighter one for the smaller scale and the heavier one for the heavier scale. The peas had hooks on them that you slide along the notches on the beam till it is level then read the weight below that notch.

You could take these scales to the field and hang from a crude tripod, tree, etc. and weight cotton or hay bales on the spot.

[J.A.F.E.]

It's been a while since I've used it but Greyhound has or had a package service. You have to drop it off at the station and pick it up at a station at the other end but they will ship (or did) pretty much anything. Engines, transmissions, big and heavy no matter as long as it fit in the bay under the passenger floor. And it was the cheapest option out there by far. The more flexible you are with schedule the cheaper.

[Aunt Phil]

OK, let me see if I can clear it up, or screw it up.

Think of the scale beam as a teeter totter.  When the sliding weight equalizes the beam is level. 

The hook is nothing more than a weight equal to the beam itself and slider at 0 position past the fulcrum.  It's kind of like the fat kid sitting closer tothe tip point on the teetertotter so the skinny kid sitting on the other end doesn't spend his day in the air.

Beam scales with different graduations on opposite sides are set up to employ adder weights on the tail end of the beam.  The scale itself is graduated on one side to weigh from 0 to 100 pounds.  If the object being weighed is more than 100 pounds, the adder weight gets hooked onto the tail and the weight of the obbect is read directly from the 200# side of the beam.  Usually a shackle is in the beam tail to hold the adder weight.

[Bus]

You don't add a weight but change the weights and they each have their own hook. The beam is usually marked with the pound weight of the pea needed for that side of the scale. Like 2 & 4, 4 & 8, etc. Here's a picture of a scale and two peas.

[Bus]

Here are some pictures from my 1918 Simmons Catalaog showing these scales. I have been calling the weights peas they call them poises. Several years ago I saw a PBS documentary on the big flood in Louisiana in the 1920's. One scene was of a flooded cotton field with one of these scales hanging on a tripod.

Bottom pic is the listing from the same catalog for steelyards with the weights like BoatGuy's

[Aunt Phil]

Different makers used different designs.

Look closely at the picture I posted and you'll notice the beam carrys a sliding weight. 
That slider accomodates weights to 100#.  For loads over 100# an additional weight is added to the shackle on the back end of the beam.