http://www.forgottenweapons.com/hotc...olving-cannon/
It is not in the OOBS since it is a pre-1900 weapon. (Though common in the worlds pre dreadnought fleets).
All 1920s-30s 37mm and 47mm anti-tank guns tend to have their genesis in these 2 rounds/calibres (37mm being usually termed the "one pounder" and the 47mm usually as "three pounder" in the turn of the century Janes Fighting Ships etc).
If it is a one-off for a scenario then a 37mm or 40mm Bofors AA gun will probably do as a substitute - just rename the unit.
Andy