HI Dave......pre 1914 0.276" - 0.280".1920' / 30's.........0.386"........Although I have one of the later at 0.380"
The above taken from direct measurement of blades.......The above sizes will be the extreme dia. of the dovetail cutter (loads of cutters on the bay, and about £5 extra to get one reground to the correct dia.)