After collecting Hameln for about a year, I've 
noticed that most of the TV series cels don't 
come with their sketches (I have yet to even 
recieve one). However I have seen a few matching 
sketches for TV cels out there, they're just not 
very common. Quite a few of my TV cels came with 
the blank sketch you mentioned. On the other 
hand, cels from the Hameln movie often do come 
with their matching sketches.

Hope this helps. ^^

ps: I'd love to see your new Hameln cels!