Random Question: What is the deal with 17?

Just a random thing that has been puzzling me for a while.  Have you ever noticed how a lot of Famicom carts have the number 17 on the back of them?  Right there in the upper left hand corner of the back label?

Whats up with that?

Not all carts have the number 17, a lot of them are just blank.  But there are no carts with, for example, the number 15 on them.  Or 16.  Or 4.  And so on.

So I was just wondering about that.  Why 17?  Anybody know?