This is a replica of the Seiryoji Shaka, a tenth-century statue produced in China after the model of a statue believed to have been carved in the presence of the historical Buddha Sakyamuni. An ink inscription on the pedestal of the statue tells us that the important Ritsu-sect priest Ninshō (1217–1303) performed its eye-opening ceremony in 1273 and that it was made by a Buddhist sculptor named Genkai. A crystal reliquary is interred within the statue’s head.  
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By legend, the Udayana Buddha was a sandalwood image carved in the presence of Śākyamuni himself. The image of Shakamuni (Skt. Śākyamuni) enshrined at Seiryōji Temple in Kyoto is believed to be a copy of that first-ever Buddha image, and this statue is a copy of the Seiryōji Shaka. An ink inscription on the pedestal tells us that this copy was dedicated in 1273 (Bun’ei 10), and that it was produced by the Buddhist sculptor Genkai (dates unknown). There is a reliquary interred within the statue’s head that appears to be made of crystal; this is inserted into metal fittings at the top and bottom. The reliquary has a hinge on one side and a clasp on the other so it can be opened and closed.
 
NAITO WataruEnglish by Mary Lewine 
Buddhist Art Paradise: Jewels of the Nara National Museum. Nara National Museum, 2021.7, p.352, no.6. 

The statue of Śākyamuni which priest Chōnen brought back from China to Seiryō-ji Temple in Kyoto had been extensively admired, therefore many works were reproduced in the Kamakura period. The present statue constitutes one of the reproductions. The ink inscription was written on the pedestal, describing the date of fabrication as well as the name of prayer and sculptor.
 
Yasuo Inamoto 
Masterworks from the Nara Buddhist Sculpture Hall at the Nara National Museum. Nara National Museum, 2010, p.142, no.123. 

The original statue of Shaka-nyorai (Śākyamuni) in Seiryō-ji temple in Kyoto, which was brought from China in 986 by Priest Chōnen (?-1016) of Tōdai-ji temple, had received special respect because of its human appearance. During the Kamakura period, influenced by the movement for the revival of classic style statues, many copies of Seiryō-ji type statues were produced and distributed all over Japan. The statue introduced in this article is one of them; it was made by the order of Priest Eison (1201-1290) in Saidai-ji temple and his colleagues. According to the inscription written in ink on the seat of the statue, this statue was made by sculptor Genkai in 1273.
Its body from head to toe was made of a single block of hinoki wood (Japanese cypress) to which hands were attached. The core of the statue is not carved out. The eye balls as well as other objects were inserted into the statue by splitting the back side of the head. The style of the sleeved garment, the curving parallel lines of the drapes, and the expression of hair, which looks like bonds of ropes, were modeled after the statue of Seiryō-ji temple. However, the sculpturing style of the Kamakura period is clearly observable in this statue. 
Masterpieces of Nara National Museum. Nara National Museum, 1993, p.28, no.15.