From AP 101B-5510-15 (Chipmunk T10 Pilot's notes) last amended 1993.
Similar, in that it mentions the opposite aileron to give a cleaner entry, but in the RAF flying training environment, this was never done. It was always an aileron neutral entry, to the point we taught the students to use both hands were on the control column to ensure it came back neutral. From the Student's Study Guide......
I gather the technique to enter the spin in the RAF changed over time. It used to be a lower speed entry which would be done close to the stall, this would probably have led to flatter more oscillatory spins which were harder to recover. The 50 knot entry is more of a 'flick' entry which is cleaner and leads to a more stable spin. The DH manual may be referring to the earler technique. This is from the Canadian flight manual published in 1957.
