The three knobs on the faceplate of the Sparton Equasonne were to the best of my knowledge the same on all models. There were 2 small knobs and 1 large central tuning knob. The two smaller knobs, outwardly appear to be the same, but have a different mechansim of attachment. The volume control knob, meant to be turned, simply pushes onto the shaft of the volume control pot by a similar but smaller mechanism to that of the large central tuning knob. The volume control knob, that pulls on and pushes off, is threaded onto the on/off switch, presumably the volume control knob attachment mechanism would have been unsuitable for the on/off switch as the whole knob would likely have pulled off the shaft each time you turned on the set!
Original knobs are occasionaly available on the internet. You could try placing a want add at this website if you like.
Alternatively Larry Bordonaro at Old Time Replications assures me he has the moulds to create reproduction replacement knobs for numerous antique radios, including the Equasonne.