The original Mark II design was prone to faults and the later Mark II* was a modified version which improved reliability.  Stoppages and jams were still frequent in these models, as the rounds slipped out of their holders and the fabric belts often stretched and tore.

Actual bore length was 39.37 calibers.

Some fifty of these guns were sold to the Italian Navy.  They were then manufactured under license by Terni (Italy) until the early 1930s.  The Terni version used a 50 round ammunition box.

Russia ordered a number of these guns from Vickers during World War I and by 1916 forty of them were on hand in the Russian Navy (20 each in the Baltic and Black Sea fleets).  In 1916 the Russian Navy ordered sixty additional guns from the Obukhov factory with at least twelve of them being delivered by 1917.  All of these weapons used 25 round belts.

There was also an earlier Mark I version, but I lack details as to what were the differences between it and the Mark II.

The data that follows is for the British version except where otherwise noted.

2-pdr on unidentified Canadian River Class destroyer ca. 1940
Library and Archives Canada Photograph MIKAN no. 3566993

40 mm/39 Vickers-Terni Model 1915 on unidentified Italian Torpedo Boat
Photograph copyrighted by Elio Andò

40 mm/39 of 342 AA battery at Vizzola Ticino (near Milan's Malpensa Airport)
Museo della Guerra di Rovereto Photograph 32/40

See Vickers Pom Pom and photograph numbers 2836 and 2837
(This is my best guess, but these last two photographs may actually be Mark VIII or Japanese guns)