Tony
Didn't say what it was, but Dave H did bring to my attention that it was a factory mistake and not intentional. Tony said there were 550, so I think you found it. Congratulations on being the first one on the forum to find it. Should have known that Terry with the Bronze Lady would know the variation.

Now we need someone with a number below 550 and someone with one above 550 to confirm it and what the spelling variation is.