Rummaging around in my gun stash the other day and came upon this. What is it?

Hint--it is a single shot 44 mag.
When I was younger and had gonads the size of grapefruits I about blew myself up with this thing. Story to follow in a day or so.
It is a shame I can't sell a gun. I could retire in style but then I wouldn't have anything to do. I still have my first from 51 years ago.