[RolandF]

To say it easy:

You say Werknummer, if you´re talking about fabrication.
You say Werkenummer, if you´re talking about the works of famous composers, writers or other artists.

Now you know that it sounds rather funny for a german, if he´s reading about Werkenummern(pl.) in this forum.

Werk is the equivalent of "works" which is used only in plural in English
Werke is the German plural of Werk, but is not relevant in this context.

The problem for foreign speakers is the double meaning of Werk, which designates not only the production site but also the product of labour in any case.

Hope this helps.

Regards
RolandF