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The "/" was maybe added because there is a big one on the grille 
On Wikipedia they mention "Volvo WhiteGMC" as make, but we could indeed use "WhiteGMC" rather than "White/GMC".
Volvo WhiteGMC was never used as make, I'm afraid that the WP page is another one written without checking...
But as the brochures show indeed Volvo and WhiteGMC together (but usually with WhiteGMC at first, see the pictures in the previous post), this could explain the mistake...
The accurate company name was Volvo GM Heavy Truck Corparation, and the 2 makes sold by it were Volvo and WhiteGMC:
(Both pictures are excerpts from the 1988 truck line brochure)