Welcome 32VAC.

Here's the feature on a typical 1960's vintage BS1363 MK plug:

[Linked Image]
[Linked Image]

Ian,
Yes, this was found on BS546 as well. Here's a 15A BS546 plug of similar vintage:
[Linked Image]
[Linked Image]

And an older 5-amp BS546 plug:
[Linked Image]
[Linked Image]