Is an elbow weighing 5 kg an old sow's elbow?

not necessarily.

Generally, the elbow of a standard adult pig weighing 12 kg is about two to three kg, and 22 kg belongs to a big pig, and the elbow is about four to five kg, which may be a boar or a sow.

The elbow is the leg meat of a pig, which is divided into the front elbow, also known as the front hoof. It has thick skin, much tendons, heavy gum and lean meat, and is often cooked with skin, so it is fat but not greasy.