The first P designates the frame class, in this case, Production. The following PP designates the engine class, in this case Pushrod Production. Hence, a production bike with pushrod engine is P-PP, and a production bike with any other engine design is P-P.
I think someone in the Norton PR department is a little confused over the PPG class. There is no such class. In other frame classes, the suffix G or F distinguishes between the gas and the fuel classes. The Production frame class doesn't have fuel classes, so doesn't include the G or F designation. They may be thinking about running in the modified class, which would be 1000 M-PG. The current record in that class with the AMA, who sanction the BUB events, is 155.848 mph, set by a Vincent last year.
Just for comparison, the current record in 1000 P-PP with SCTA, the other Bonneville sanctioning organization, is 144.626, set by a Buell in 2003, and for 1000 M-PG it's 157.98 mph, set by a Guzzi in 2009.
Ken