I feel compelled to point out that I am pretty sure that niether of these options is actually correct for a P11, but rather for the later Rangers. There are loads of people on here who will know for sure.
You wanting OEM? https://www.ebay.com/itm/NOS-NORTON-P11-RANGER-REAR-TAIL-LIGHT-ASSEMBLY-/142702618437 (price is a little high for my tastes)
I'm going with a generic bobber tail light on mine. Let some future owner worry about getting the correct one, which frankdamp says will fall off anyway.