Got this to replace a 12V adapter for a battery charger that went south, and figured I might as well have something that will stand in for any other failures in future. It works for that battery charger just fine.
Choosing the polarity was, as others have said, not entirely obvious. There were no instructions either in the packaging or on the adapter itself.
There is an arrow on the socket that accepts the tips, and an arrow on each side of the tip, with a little diagram showing the polarity. I lined up the chosen polarity's arrow on the tip with the arrow on the socket, and all worked just fine.
My device is from 'Mercury' [...], stock code 661.406UK [...], jacks, voltage and current as described. It's 88 x 51 x 47mm, and 235g in weight (less than advertised, win).