Those would be the original non-Monitor K 141 (Monitors had non-Sextett drivers and different colors). Looking at the headband, I might make a guess that they may be from earlier stages of production.
Would you mind doing a sine sweep on them? There's a review and frequency response graph there: http://wiki.faust3d.com/wiki/index.php?title=Image:AKGk141review79.jpg. The reviewer makes a specific note about the valley between 4 and 7 kHz, and it should be easily heard if it's there. For some reason my vintage K 141 doesn't have it, so I suspect AKG either tweaked the 141 during production or mine has been serviced with Monitor drivers.
As a side note, as the reviewer notes about the reversed phase on the 141, has someone actually tried reversing ('correcting') the phase on the K 141 to see if the sound improves? Mine are indeed wired in reverse, but so are a lot of vintage AKG phones - for whatever reason.
Edited by vid - 9/14/12 at 4:42am