Andel Left Crank Arm, Offset Square
Andel is a high end manufacturer of OEM cranks. They make the All-City branded square taper track cranks, as well as cranks under their own name and others'. Unlike many other common square taper cranks, the square on these is offset from normal. See the photo, or imagine that when the arm is horizontal, the square mounting hole is oriented as a square, not a diamond.
These arms are beefy yet light, and the Q factor is narrow. The pedals end up hugging the frame nicely.