<Most flat roofs of that era and later in the Midwest are "tar paper".
More accurately; roofing felts tarred over in several layers. I have
crawled over many and on hot days, had the gooey tar on as many pairs of
shoes. The felts do not have a granulated top coat, just goo.>
I think a lot of what is being car "tar paper" roofs is really rolled roofing.
Think 30" wide rolls of granulated roofing material. It would look like tar paper in many photos.
Very common on secondary farm buildings in the 1950's where I grew up
