Are we talking Brake hp, SAE hp or RAC hp?
Obviously recent engines can be run on a dyno to measure the hp, but how was the hp actually measured on these earlier engines?
Don't forget the 'norm' in earlier times was simply to calculate it using the RAC method (really for taxation) which bore little resemblance to the actual output with (for instance) an Austin 'Big' 7 of 900cc rated at 7.99 RAC hp but actually giving 25 bhp when measured on a dyno.