You are looking at a dust cover cap in the colour of choice for your leisure cable 16amp hookup plug!
These are solid, robust 3d printed pieces. Great for keeping your connector clean, although we can't guarantee they are waterproof in any way, it is definitely better having one of these than some exposed pins!
Great solution for cables as the majority of ones these days don't come with caps. The cap itself does have a twist 'lock' feature for the raised part at the top of the connector, but obviously this can vary with connectors how tight this will actually be - but the main groove should definitely accommodate the majority of 16amp blue ends on the market.
Note: Cable not included
Dimensions: 43mm diameter for connector to enter (apparently some 16A cables are slightly larger, but this fits every one we have as they are all around 42.5-42.8mm!)