As your text states, the Revisionsstelle is a body (=Organ) of the company, and therefore neiter a company in its own right nor an authority (this sounds as if it was part of IRS). This is obviously a Swiss company, as the term Revisionsstelle is not used under German law, but is the technical term in accordance with art. 727 of the Swiss Obligationenrecht, which prescribes the mandatory bodies of companies.

The function within a company encharged with internal auditing is called audit(ing) department, see example below:

AUDIT OFFICER
AUDITING DEPARTMENT
$48,000 to $68,000 & Bonus
Global bank has an outstanding opportunity available within the audit department for an internal audit officer. Participate in the
execution of internal audit work, to include planning, field work, reporting and follow up in a manner consistent with approved policies and procedures of the department. Conduct special audits, special investigations and risk and internal control assessments. Represent the department in various activities or projects, perform consultative services to management,
coordinate with regulatory examiners, public accountants, consultants and head office. Individual must have a Finance, Accounting or IT degree with a minimum of 12 credit hours completed in accounting courses. Minimum of 3-6 years experience with a financial institution or public accounting firm. Certification as a CPA, CIA or CISA completed or in progress strongly
encouraged.