[Elecraft] crystal envelope number - how to use?

[Wolf-Ruediger Juergens]

In the manual (rev. C) page 50, second point is mentioned that the number
(identifier for tested frequency) from the crystal envelope should be 
noted in the manual for later using. (aligning CW filter).
How can the number be used? In the aligning part is no reference
to the number.

72 de Wolf, DL2WRJ