The repair kits and pistons for early calipers with imperial thread can be of two types and they are not interchangeable. The later type kit (part no. 519731) differs from the early kit (part no. 601960) in using a steel clip to retain the dust shield on the caliper body. For further reference please refer to the illustrations.
The calipers 307977 and 307976 can be replaced directly by either the later imperial threaded items or by the metric threaded ones if the appropriate brake pipe and caliper mounting bolts are used.
From commission nos. CP76095 and CC81079. Metric threads.