Unfitted No1 mk3 magazines?

[Lucite]

I have a no1 mk3 rifle was having bolt problems and when I took it to the gunsmith he said it was due to an improperly fitted magazine and he said to buy an unfitted magazine and he could fit it. I'm just wondering if there is such a thing as an unfitted magazine or if its just find one that's the best fit for the rifle and have that fitted.