The signature of Prince Damrong Rajanubhab (1862–1943) of Siam (now Thailand), reading "ดำรงราชานุภาพ" ("Damrong Rajanubhab").
Extracted from Page 12 of this file: File:จุลยุทธการวงศ์ ผูก ๒ (๒๔๖๓).pdf; the source of that file (as stated in the description of the file) is as follows:

Somdet Phra Phonnarat (Kaeo). (1920). Chunlayutthakarawong Phuk Song จุลยุทธการวงศ์ ผูก ๒ [Chronicle of the Minor Wars, Second Bundle] [Phra Yannawichit (Sit Lotchananon), Trans.]. Bangkok: Sophon Phiphat Thanakon Printing House. [Printed for distribution at the funeral of Phraya Phahonyothin (Nop Phahonyithin), held in 2463 BE, being the Year of the Monkey or 1920 CE].
Bitterschoko (for this file); the signature itself belongs to Prince Damrong (1862–1943) of Siam (present-day Thailand).