The spring ensures the return of the siphon’s push lever to its original position, while the clamping screw clamps the spring and holds it in place. The locking screw on early siphons is made of aluminium, while the aftermarket siphon heads have a black plastic part.
The clamping screw at the top of the siphon head can be a hit or miss. Sometimes it can be easily unscrewed by hand, but sometimes it takes a lot of force to get it to move. A pipe wrench always helps, but it’s better used carefully, as the clamping screw is a visible part and there is no newly-made equivalent. Similarly, care should be taken not to damage the plastic of the head during maintenance.