Thanks for your reply. I wonder if there is a general rule of thumb that the ones with impedance stamped on the outside are always 300 ohm, and the ones with the value stamped behind the earpads are always 600 ohm (can't really determine from your picture if it's 300 or 600).
Since I'm looking to buy through an online auction site I'm thinking it might too much to ask from the seller to dismantle the headphones to find out, and if he or she do not speak English it would be hard to explain anyway. What I'm getting at is if there is a way to find out the impedance by just looking at pictures of the exterior (provided such pictures are available).
I've seen both 300 and 600 ohm versions of Ref.1 marked on the outside. The earliest units made were all 600 ohms, so there was no need for external marking when they were all the same impedance. I believe the outer marking only started when both impedance versions became available. I'm not sure whether Sennheiser ever made 300 ohm Ref.1 units that were unmarked on the outside, but I haven't seen one. The Ref.2 didn't need outside marking because they were all 300 ohm.
By the way, the unit in my picture is 300 ohms.