According to Browne Subh-i-Azal was the successor of the Bab

Browne attached great significance to this letter and called it 'a document of great historical interest, viz. the appointment of Subh-i-Ezel [Mirza Yahya Azal] by the Bab as his successor'. Making no distinction between the rank of a successor and that of a vicegerent, Browne continues: 'This document furnishes us with the grounds whereon Subh-i-Ezel's claims to be the Bab's vicegerent are based.' Elsewhere, Browne states that the appointment was made explicitly. Browne evidently regards the role of a successor or vicegerent as that of a kind of vali, who leads the community until the coming of the new Manifestation.

(Making The Crooked Straight by Udo Schaefer, Nicola Towfigh, Ulrich Gollmer, p. 633)