Yes, Alfred, sequential numbers.
From a statistical point of view, that is as likely as any other sequence.
As further proof that my car has 519 and 531, here is a picture of my data plate.
Very strange about the pricing card. I have no idea why the Grand Prix is not mentioned. I look at data plates whenever I am checking out a car and I have seen 519 out in the wild, so Ralph's and my car are not the only '70 Pagodas that have Grand Prix radios.
What I was hoping for was to confirm it was the mono available for '70 280 SLs and what the button markings and face plate markings look like. But, we are far from understanding that.
Attached are also pictures of Ralph's period replacement radio and mine. The file names indicate which is which. Ours look very similar or the same except for Ralph's goes to 106 on the FM dial and mine goes to 108.
And, with the last attachment (green car), you will see that sometimes the dial has the LMKU on the right side of the dial, whereas, mine and Ralph's are on the left side of the dial.