L6 - in-Line 6
or
I6 - In-line 6

https://en.wikipedia.org/wiki/Straight-six_engine
https://en.wikipedia.org/wiki/Straight-six_engine#Balance_and_smoothness
https://www.quora.com/What-are-the-pros-and-cons-of-V6-and-inline-6-engines

The flathead L Head engine was a different design.

"A straight-six doesn't need split crankpins, balance shafts, or big counterweights, because each of its cylinders has a twin that's doing the opposite thing, at the same time and in the same plane, canceling out the other's forces. That lack of internal dissonance gives the same perfect balance as a V12. There's a reason museum-piece marques like Rolls-Royce, Mercedes-Benz, Jaguar, Aston Martin, and Alfa Romeo earned their reputations with inline-sixes."

"It's a dream aligned in one straight line."