Same steel, some slightly different details like the wrap around seatstays on the Grand Prix. From having both, in a given year the GP's workmanship might be slightly better, depending on the plant that made it. But these were workmanlike machines to begin with.
W,G,or R coded Grand Prix's would likely have a slightly higher quality level. But I have an "N" Nottingham made GP and it's a perfectly serviceable, comfortable ride.
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I have a white PX-10, a Green Dawes Galaxy and an Orange Falcon, now I'm done.