Name of this toy

I played with this building type toy when I was a kid. Maybe someone can help
me remember what the name of it was.
It had 2 thick white pieces of masonite. In each piece there were about 6 or 8
holes drilled evenly apart. Into these holes were placed little black plastic
thingies that looked like steel girders. These were used to make buildings. It
was a building set of some sort. After the building was completed (the skeleton
of the building looked like a real steel girder building.) you then attached
these index card sized transparency film thingies to the plastic girders. They
had designs on them that looked like windows. When you were done building you
and what looked like a real skyscraper. I played with this in between 1974-1976
(?) I can't remember for the life of me what this was called. ANY help in
naming this would be greatly appreciated. Pictures would be cool too. Thanks
for any help provided.

Regards,
Jeff