As you can see on the attached screenshot, the Unit #, let alone the book page number, are nowhere to be seen on the Lingco page displaying a given drill. The Unit # is sometimes needed (as today, when referencing for a student, and noting a misprint for the publisher), so it can confuse both myself and my students---we have so many #s to keep track of!

So, I'm proposing that all Drills and Listening Exercises for Alif Baa' have the unit #, as:

Listening Exercise 5 (Unit 4)