Rodenstock Rodagon 135mm f/5.6 enlarging lens, 39mm thread mount, retaining ring included. Serial #10064265 - made in Germany.
Covers all sizes up to and including 4x5", and is pretty well regarded - a quick google turns up a lot of positive posts. It's a 6 element lens in 4 groups, by my understanding.

I purchased this privately here on APUG a few years ago, and only used it a few times in my home darkroom before I moved into the city to attend university. I use the university's 4x5" enlarger, so this has actually been sitting in my shelf, safely sealed inside a...um, well, if you must know, inside a glad sandwich bag...for a couple years. Certainly has kept the dust off of it.

Glass is clean and clear, delivered lovely prints when I used it, aperture is perfect and snappy.

$80AUD Including Australia wide postage. International postage at cost. I accept bank deposit and PayPal, but I prefer bank deposit because of PayPal's fees.