Actual power for 398 cc

Roger wrote: The engine has a 3 h.p. Treasury rating for taxation purposes and this is based on capacity as a 500cc engine would be 3 1/2 h.p.

I try use RAC formula (d^2 * n)/2.5

Result:
(2.72^2 * 2)/2.5=5.9

5.9 per engine
2.95 per cylinder

"ABC 3 hp" = "3 HP per cylinder" ?