The Model 51 mixer was made between 1951 and 1963.  Because the original cups were plastic, not many survived over the last 50 years.
The cup holds a little more than a pint.  It has a double bottom with ring so it doesn't slip off the bottom holder.
Do not try to use with the taller single mixers. Those cups are 6 7/8 inches tall.
This cup , at 5 inches high, is a little shorter than the original, however, it is the closest cup we can find.  It works fine, but we recommend holding on to the cup until the mix starts to blend.  Then you can release the cup to work on its own.
Stainless steel.  Imported.
Weight:  8.3 oz.  (.52 lbs)
New
It's a good idea to check the feet on your mixer, as they become brittle and should be replaced. Otherwise, when in use deteriorated feet can cause damage to and slippage on the counter top.