Spiral knurl! That's a new one on me!
Cue the folks on the shop-floor at Durhum Duplex tutting at the new Stores Manager for ordering the wrong knurling while the Production Controller is pointing at his watch muttering, "time is money, men!" ... and so they fitted the spiral knurling and ran a few miles of rod through it before replacing with the usual knurling once the Stores Man had been given his orders.
No, that's an interesting twist (ha!) ... I love how vintage that we must have seen a thousand times keeps giving us surprises decade after decade. These Durhams really were damn good razors with the period blades, which do still hone up. I know I sent you a small number of modern pathology blades (which I trust made it through various customs), but they're nothing like the old blades.