What is the value of the king single shot no21 air rifle?

Like all antiques the value is determined by the current condition of the rifle. You have provided no information about the condition, so it is not possible to give you an answer. You need to give it some sort of rating like poor, fair, good, very good or Excellent. you also need to state if it still is working. Tell me about how it looks, is it very warn or broken?

I can tell you this. The KING (Markham-King Mfg) model Single shot no 21 was made in two variations from 1907-1932, and has considerable value.