The VI507 is a spark gap designed to break-down at 1250 volts. Inside is a small bead of mercury that provides the gas filling. The AM signifies that this was made for the UK Air Ministry and must date to before 1941 as after that date it was referred to as the CV1507.
The electrodes are sheathed in glass along their whole length, just the ends are exposed to make the gap.
Mark Mallison supplied this image of another VI507 that has the up arrow but not the AM and crown.
The thin glass tube envelope is 19 mm in diameter, and including the pins is 49 mm tall.