I had unlimited access to twine in those days,and the winters were quite long , we had no telly at the time,I have made one or two for others but my go to knife for fishing and filleting is the swiss Inox knife keeps a good edge , but I found a Lagoiulle folder and it makes a good filleter ( for sea fish), a local sawyer showed me a section through a plank from a tree from Inverness, with a pocket knife in it , we reckoned it must have been stuck there 60 years back and forgotten about the tree had completely engulfed it,luckily it hadn't managed to damage the saw blade, easgach 1