I think that Blevins is showing the wrong building or the wrong end of the records building..it's all really fuzzy... but I found it.
Go to Google maps and look up 498 Elm St Dallas Texas and it will take you to the point at Dealey Plaza where you can clearly pan up to the roof of the Records Bldg and then zoom in.
The light posts are there at all the corners but we wanted one that bends to the right over the possible shooters head. You will find it by panning to the right along the west side of the roof [that would be to the southwest corner] this is between the Records Bldg and the Old County Jail.
There it is...a rooftop lamp post identical in every way to that shadow.
I wonder why they put lights up there?