Got them from Polxmax;

http://www.polymax.co.uk/acatalog/NB...9_-_BS395.html

the ones i have are the BS313 size, i measured the grove on mine at 4.8mm which is 3/16, and the diameter and bought the 3/16 section rings (which are actually 5.33mm wide) they sealed perfectly, and work ok, could do with the next size up in diameter really as the breaking force is not enough,

Rich.