The one listed from LMC truck is not a 1958-1959 gmc park light. It is in fact a cheaply reproduced 1955-1957 chevy park light. It DOES NOT INTERCHANGE with the GMC.
I wondered about that back when I saw the catalog, so let me ask a question in this area. What is the difference, the size? I ask because my fleet option 59 has similar lights and I don't know what GMC used although I've been told it was the reverse lamp lense on a step side.