58 Pa. Code § 185.55
If any registered stockholder provided for under this subchapter is not or is not to be the sole beneficial owner of such stock, the registered stockholder shall set forth in the affidavit required under this subchapter a true and complete copy of all terms of the agreement pursuant to which the stock is to be so held by the registered stockholder, including a detailed statement of the interest therein of each person who is to have any interest in such stock. No person may hold stock as a registered stockholder for the benefit of any other person unless a duly executed agreement in writing exists setting forth all of the respective rights of the registered stockholder and the beneficial owner or owners as well as all of the terms of the agreement between the registered stockholder and such beneficial owner or owners as there may be.
58 Pa. Code § 185.55
This section cited in 58 Pa. Code § 185.58 (relating to change of status of affiants who are beneficial owners).