I would think they order the receiver with magazine and add barrel and stock. the action was the heart and making barrels and stock would be much simpler for vickers.
believe this was just a way to find some work after the great war when stocks on military guns were so high it last to the summer of 1940.
you wonder this rifle is so rare today because a big firm like vickers was probably able to make this rifle in a great ammount for a good price with the machine park they had.