Well, the cup design and the R.P.D. abbreviation was what sent some tingling down my spine - an OEM Fostex "Regulated Phase Dynamic"? But as far as I know, there never was an 150 ohm Fostex (that impedance is typical to Yamaha and PMB).
And once the lid is removed, we can see the driver!
Yes, most likely the ubiquitous Sawafuji/Audio-Technica/Pro 30 driver. It's glued in place. The construction of the cup makes modifying quite easy - it only takes a minute to test different types of damping.
khbaur has an Audio Technica driver that is 150 ohm in his rare ATH-M2 I think it was called. It's posted up in here. The only 150 one i have heard of in that style is the ATH, well also the HP-50 type but that doesn't have the housing ring around the outside does it ? ( don't think so but I forget ATM ).
Can you feel how many holes there are underneath the damping there? Or shine an LED flashlight through . It would narrow it down.
Edited by nick n - 4/3/13 at 5:39pm