http://opticsplanet.net/pentax-eyepiece-xo-20.html

It looks to me like it is a Pentax XO-5 (Duh)

I especially like the eyerelief:

XO 5

Lens Structu 5 elements
3 groups

Focal Length: 5.10mm

Apparent Field of View: 44°

Eye Relief: 3.6mm

jon