It is not sterling - Sterling is 925 and 3 towers is (at least) 830. So silver it is.
Or 13 1/4 lod (lod is the old term used for centuries) 3 towers is used since 1608 and yours if from 1950 - you can see it from 50.
13 1/4 out of 16 - where 16 is 1000.
SJ is the guardein. look here
http://www.925-1000.com/forum/viewtopic.php?t=5758" onclick="window.open(this.href);return false;
Guardein is Danish for the man who controlled the silver.
The third mark I cannot identify. Try to describe it. This is the mark of the maker.