Hairspring guards are designed to prevent the coils of the hairspring from jumping over another coil in the hairspring. When this happens the movement will start to run at a much faster rate often gaining several hours per day. Hairspring guards are most often found in early Submariner or GMT Master Rolex watches.
These early Rolex movements used a blue colored hairspring rather than the white alloy hairspring found in the later models. The movements with the blue hairspring have a slower beat per hour vs the white alloy hairsprings.
Below are some pictures of the hairspring guard found in a Rolex movement 1570. The movement serial number is D905606.
This shows the hairspring guard which is the brass wire that is over the balance at the left.
Here is another view showing the hairspring guard extending over the hairspring to the center of the balance wheel.
This view shows the hairspring guard over the balance. Most of these movements have a hole for installing the guard. The base of the guard has a notch so that it can be rotated away from the hairspring in order to remove the balance for service.
A side view of the Rolex 1570 movement showing the hairspring guard over the balance wheel and hairspring.
This is the dial side of the Rolex 1570 movement showing the date wheel and dial washer.
These Rolex movements are some of the best designed movements that Rolex manufactured. They are very durable and rugged and will last many years and provide good timekeeping abilities if properly maintained. If the watch is to be used in a rough environment where it might be subjected to shocks and extreme bumps it may be necessary to have the hairspring guard installed to prevent the coils from jumping over one another.