There are two Optiper versions. The first has the name written between the two lenses, and has no small opening at about the 5 o'clock position outside of the wind lever hump. The second has the name on a plate that rotates around the taking lens, and has the small opening at 5 o'clock.
The small opening is a shutter cocked indicator window, by the way. I never use it but hey...
The early Optiper and the Seikosha have ten aperture blades. Later Optipers and Citizens have eight aperture blades.
The Citizen has a strange cocking mechanism that requires a bit more pressure than the other models. In practical use this is not important, but if you had a Citizen version and a Seikosha version in hand you would feel it.
All said, Brian Shaw's comment is the most useful one. Condition!