connor - looks like your E pump is new enough so you have both options. either swap the top section of your L pump onto your E or swap the arms over
my E and L are different
L on left E on right
difrangia - i've read in another thread that the E and J pumps are the same for 320 520 and 521's. and as Connor has said the L pump does mount to the E block. i have a pump with a primer lever and one without and both came off of E's.
as for the hybrid lever - fabrication is not my strong point. the L arm is solid and the E arm is made from three separate pieces connected with the rivets/studs.
the L pump has a slightly lower pivot point than the E as well
the one-way valves and their gaskets are exactly the same in the L and E and can be interchanged.