I did a little bit of research and the earliest "hemi" I can find is the 1903 Welch, built by Chelsea Mfg. Co., Chelsea, Michigan. It had a 20-hp, two-cylinder engine with overhead valves and hemispherical combustion chambers. The company moved to Pontiac, Michigan in 1904 and began producing larger cars with 36-hp four cylinder engines, still with overhead valves and hemi-heads. The 1906 engine had a 4.5" stroke and 5" bore with a single overhead camshaft and had one of the first water pumps driven by the fan belt.