That is a rare lens, you are very lucky.
It's an early "T" lens- the inner coatings are soft, if you do open it up- be very careful. The Sonnar 5cm F1.5 of that SN range use set screws hidden under the aperture ring to hold the namering in place, and a set screw to hold the rear section in.
I look forward to seeing some shots taken with it.