But then you would have to put a number on every work just in case you extend the copyright later. Otherwise you might have a 35-year-old work and have no idea if its copyright has been extended because it was not originally printed with a number.

>> Just give it a number and make protection contingent upon publication of same with the work.

> you might have a 35-year-old work and have no idea if its copyright has been extended because it was not originally printed with a number

No... if protection is contingent on publication of the number along with the work, then a work printed without a number is unprotected. There is no concept of "extending copyright" on a work that isn't even copyrighted.