O.K. Here's what I found on the internet.
"L-head (flathead) refers to the pushrod valvetrain configuration in which the valves are placed in the engine block beside the pistons. The design was common on early engine designs, but has since fallen from use."
"The line of intakes along the side of the engine lead to the name L-head, due to the cylinders having the shape of an upside-down L. This configuration is also known as sidevalve, as the valves are located beside the cylinders."
"Although L-head inline 4 and 6 cylinder engines were frequently used for automobiles, tractors, etc., the best known L-head automotive engine is the early 20th century Ford V-8, which has both sets of valves (intake and exhaust) located on the inside of the "Vee," and which are all operated by a single camshaft located above the crankshaft. A serious concern is that the exhaust follows a more complicated path to leave the engine. This virtually guarantees that the engine will need an unusually large radiator to avoid overheating under sustained heavy use."
"Due to the heating and efficiency problems, L-head engines fell from high power uses such as aircraft engines fairly quickly, prior to World War I. They lived on for some time in the automotive world and were used in the World War II Jeep, for instance."
I also found that L-head type engines including V-8's were used in Cadillacs at least through 1929.
