The Answer

Yes this dog had a urethra FULL of stones - from where it exits the pelvic canal to just before the "os penis" - which is, yes, a small bone IN the penis of male dogs and a great area for stones to lodge (the urethra cannot expand as it passes through this U shaped little bone.)

What was even more amazing was this dog somehow could still pee and had a small bladder. Of course he was very uncomfortable.

In cases like this we try to flush the stones back into the bladder where it's much easier to extract them. This is unlikely in a case of this severity and indeed, the surgeon had to open the urethra up to get some of them out. This is tough because it's already a small tube and can very easily stricture or get much narrower if sutured so we often leave it open to heal with a little time. They do quite well. In the meantime, the dog urinates out of the little hole there.  

This dog is doing very well post - op!

PS. For the person with the answer of a constipation issue : that never "presented" itself and was never an issue after surgery. However, your assessment on the radiograph was not wrong.