Why early Royal Navy OPVs carried surplus Bofors

Why did early Royal Navy offshore patrol vessels carry 40 mm Bofors guns?

The guns were surplus, cheap, and available; purchasing new equipment for OPVs was difficult to justify in that period.

Surplus 40 mm Bofors guns were available and cheap to fit to the early patrol vessels. Obtaining a newly purchased armament for OPVs in that period is described as particularly difficult. Their weapon fit therefore reflected the equipment already on hand and the low priority assigned to buying new kit for patrol ships.