An E27 cap is defined by a thread, a shell diameter, a contact position, and an overall height, and each of those needs its own check. Buying a single "E27 gauge" and calling the inspection complete is common and wrong: the thread gauge proves the cap will start in the holder, but it says nothing about whether the centre contact will actually touch.
First, thread gauging with the go and no-go members for the cap thread. Second, shell diameter, because an oversize shell will jam and an undersize shell will not grip. Third, cap height and the position of the contact relative to the shell face, which together determine contact pressure. Fourth, a contact-making gauge, which is a functional check in a simulated holder and is the only one that proves the lamp will light.
A cap can pass every dimensional gauge and still fail to make contact, because the failure is a combination: a slightly short shell plus a slightly recessed contact plus a holder with a slightly deep socket. The contact-making gauge reproduces that worst case deliberately, which is why it exists as an artefact rather than as a drawing dimension.
Keep the set together in one case with the calibration certificates, and record the sheet amendment of each member. An E27 kit whose thread gauge is on an old amendment and whose contact gauge is current will give contradictory results on the same lot, and the operator will trust whichever one is easier to pass.
