Vernon Dursley and the Seal of Hogwarts

[David]

No: HPFGUIDX 161923

There has been a lot chat about Vernon Dursley and there is one thing 
I noticed and that was in the movie and the book, Vernon's eyes lit up 
when he saw the wax seal of Hogwarts and he knew what it was. 

My question is how did he know? Did Petunia describe it to him at one 
time or did some one in Vernon's family go to Hogwarts?

dragonkeeper