What animal is this bone from?

Answer

Dear Frank,

the photo shows the underside of a right humerus (humerus) of a bovine. This is the side on which this leg, together with the bones of the forearm, radius (radius) and ulna (ulna), forms the elbow joint. The side of the bone away from the ruler contains a number of cylindrical articular surfaces, separated by ridges, that form part of this articulating joint of the elbow. The deep pit on the other side accommodates the hook-shaped protrusion of the ulna (the olecranon) when the elbow is fully extended (fossa olecrani).

Since you can so easily place this bone upright, upside down on the table, it will probably have been sawn just above this joint. So most of the bone is missing.