Here are two very informative sites about the process. Link
The casting 3232-656C is correct for the (English AP) Lockheed servo fitted as original equipment to the 1750 Berlina GT Veloce & Spider Veloce, listed as January 1968 onwards.
Repair kits for this type are still on the market. Pls see www.powertrackbrakes.co.uk who are very supportive. The rubber membram should say 3818-411 and the main piston is 5/6 or 15.875mm. It is a good idea to get a new air valve too and replace it at the same time.
Please scroll down to see the step-by-step procedure.