The Headspace Gauge - or as it is more commonly known - The Debubbler.
An insignificant but essential piece of canning kit. The 'crocodile teeth' shaped end measures the headspace at the top of the jar. Just place the corresponding step on the rim of the jar and then fill to the bottom of the Debubbler. This will ensure that you never overfill and therefore do not suffer from siphoning of the contents from the jar during processing.
Turn the gauge the other way up and you use the pointy end to disperse all those pesky air bubbles that can get trapped in the jar when you are filling - again helping to prevent siphoning.
A simple piece of kit but very important!