Unusual OBS circular ink stamp

I wonder if you can help me? I have a question for you.
An eBay seller has a greek pressing of The Damned album, Machine Gun Etiquette, with an unusual ink stamp on side 1 of the record label and rear sleeve. It is a small blue circle with blue letters OBS inside the circle..Do you know what this means? The seller seems to think its a promo stamp? I would like to know before buying.
I have photos ..
Many thanks for your help.
Gary

Doesn't sound like a promo anything. Maybe someone's owner mark?