The signature of Prince Damrong Rajanubhab (1862–1943) of Siam (now Thailand), reading "ดำรงราชานุภาพ." ("Damrong Rajanubhab.").
Extracted from Page 38 of this file: File:พระราชพงษาวดาร ฉบับพระราชหัดถเลขา ภาค ๑ (๒๔๕๕).pdf; the source of that file (as stated in the description of the file) is as follows:

"Phraratchaphongsawadan Chabap Phraratchahatthalekha Phak Nueng พระราชพงษาวดาร ฉบับพระราชหัดถเลขา ภาค ๑ [Royal Chronicle: Royal Autograph Version, Volume 1]. (1912). Bangkok: n.p. [Printed for distribution at the funerals of Princesses Woraset Suda, Ubon Rat Nari Nak, Chanthra Saratsawan, and Yaowaman Naruemon, held at the Royal Plaza in 1912.]
The pdf file is digitised and published by the Office of Academic Resources, Chulalongkorn University."
Bitterschoko (for this file); the signature itself belongs to Prince Damrong (1862–1943) of Siam (present-day Thailand).