Orbit Research Orbit Reader 20 User Manual page 50

Orbit Reader 20 has two holes on the lower-back corners of the device, one
on each corner. The holes are there to provide a means to attach a strap to
the device with the use of key rings. The key rings can be 10-40mm in
diameter and up to 3mm in thickness. The strap and key rings are not
included with the purchase of the Orbit Reader 20.