What is the brand and model of the sextant?
It is a Celestaire Astra 2.
Source: viewed image full size.
According to a conference paper from the 2019 the sextant is an Astra III.
The paper is HOLT, Greg N.; WOOD, Brandon A. Sextant Navigation on the International Space Station: A human space exploration demo. 42nd Annual AAS Guidance, Navigation and Control Conference, Breckenridge, CO. 2019 February 3; AAS 19-06415 pp.
The quote is:
The flight instrument was a Celestaire Astra III Professional hybrid brass/aluminum sextant, serial number 1183118.