Clearly the 300/600 does not apply to sub 6/12 airguns ('cos lots of air rifles fall short of the 300/600 rule) and it appears that the law does not define "pistol" or "rifle" so it will come down to what a judge/jury might decide. There are probably not very many airguns where it is difficult to tell the difference. Designed to be used one handed = pistol. Designed to be fired from the shoulder = rifle. If there are any airguns where this is not obvious then it would probably fall to the manufacturer's description as a guide. The lack of clarity on this matter strongly suggests that it has not been an issue in the past.
It would be possible to fit a shoulder stock to a (former) pistol. My expectation would be that if this is easily removable then it remains a pistol, but if it is necessary to remove screws to get it off then it has been converted to a rifle. And vice versa.
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TANSTAAFL