The first mass produced American automobile powered by a 2 cyl Hemi engine was the Welch Motor Car . First built in 1903 by the Chelsea Mfg. Co. The engine was designed and built by A.R. Welsh and his brothers. In 1909 The Welsh was bought by GM who produced the Welch-Detroit. In 1906 Welsh produced a four cyl Hemi and in 1909 they built a six cyl Hemi engine.
The Welsh was a " massed produced vehicle as much as any other "massed produced vehicle of it's time. Not as big as Ford or Chevy but a viable auto company as any.
Below is the first "true " Chrysler Hemi but it was developed in conjunction with Continental Engines. makes you wonder if Chrysler ever did build there own engine. Hmmmm?