Bicycles and different machines need different size and type of crank arm. This is because riding machines are different as well. In a scenario where the cyclist uses a 50/34 crankset, the arm needs to be a size that is allowed by the crank arm size chart. If the cyclist uses a 54/36 crankset, the arm size would need to match. So, if you are looking for a rare Hexagon/6 sided spindle left crank arm to fit a 55/36 crankset, the arm you are looking for is the MR-1A3.