Specification:

Whether standard parts：standard part

Material: Aluminum

Modulus: 0.5

Out Diameter:31mm

Inner Hole Diameter: 6mm

Teeth width:5mm

Pitch:1.57mm

Teeth Number: 60

http://vi.raptor.ebaydesc.com/ws/eBayISAPI.dll?ViewItemDescV4&item=141938661758&category=260272&pm=1&ds=0&t=1588332444000&ver=0

https://www.astroshop.eu/drive-motors/skywatcher-eq-2-ra-motor-drive/p,1532#tab_bar_2_select

1.25″ 90°Zenith /plane Mirror for Eyepiece Astronomical Telescope Accessories

Source: 1.25″ 90°Zenith /plane Mirror for Eyepiece Astronomical Telescope Accessories | eBay