Might be a cellular (two piece) deck with 16 gage deck as pieces. But if it's only 1 1/2 inch deep, it won't span 24 ft.
Check SlideRuleEra's website (I don't have his address here) for Robertson Catalogue Information. Maybe it's 4 1/2 inches deep.
I suspect the 1 1/2D note may have been a typo. It probably was intended to be a 6" deep X 16 gage deck. I am going to have the contractor field verify dimensions