[vanir]

Vickers-Armstrong developed the British Ordinance 2-pounder anti-tank gun and I should think with the same armour penetration as the field weapon it probably uses its breech. The barrel looks all new. A loader would have to have been adapted, perhaps this is from the AA weapon?
The AA gun was developed for a high rate of fire and good charge weight, it doesn't have quite near the breech pressures, but like the Oerlikon they are much better to modify for aerial weapons. In this case however, I'm fairly certain it was taken from the field weapon cartridge/breech combination, like the Molins.