The appellant or representative may request that the Board obtain an opinion under Rule 901 (Sec. 20.901 of this part). The request must be in writing. It will be granted upon a showing of good cause, such as the identification of a complex or controversial medical or legal issue involved in the appeal which warrants such an opinion. (Authority: 38 U.S.C. 5107(a), 7102(c), 7104(c), 7109)