I think they are primarily found on cars originally titled in states that require a front and rear plate. They would have been installed at the dealer before the car was delivered to the first owner. My car came from NJ... and the front bracket is an original factory piece.
Mmmm not sure...I bought my GN brand new in '87 and it did come with the front license plate bracket (it was in a package in the trunk) although we do not have front license plates in Quebec. So this makes me think that all GN's must have had the bracket when new, but the only explanation I can think of, is that some dealers did not install them unless needed in their area and threw them away...
Claude.