According to App 1 to JAR-FCL 1.075, only an Examiner or the Authority is permitted to sign the revalidation process. An Instructor is not so authorised.
Then this must be one of the few german differences to the "universal" JARs. If you read any German then this form may be of interest by which an instructor or examiner notifies the authority that he has revalidated an SEP classrating in handwriting on the applicants license:
Luftfahrt Bundesamt - Lizenzierung - Nachweis Handeintrag SEP
It references FCL 1.024(a) (german) that seems to be missing from the english FCLs. So much for international standardisation!