The 2" shell was actually developed years before the 2" gun. Light loads for standard Brit game guns. Then, during the between the wars years when competition was tough and new guns weren't selling all that well, someone came up with the idea of a gun specifically designed for the short shells. Sort of like the Churchill XXV: at least part marketing ploy, to offer something the competition didn't have.
The early 2" guns underwent the same standard British proof as 2 1/2" guns. Later--during the 50's, I believe--the proofhouses established a lower proof standard for 2" guns. All the Spanish 2" guns made more recently were subjected to the CIP standard proof.
