10 Sterling

10 Sterling are mostly a jewellery outfit as far as I can tell, however they do a range of "Neck Chains & Cords" that work quite well for subtle collars I think.  Many of these are reasonably high-end stuff, made from silver and titanium, but some of the more basic lines also feature neat tricks like magnetic links to hold them shut.  They also stock more collar-like things including stainless steel segmented collars and steel neck chains. If you're looking for more masculine subtle day collars many of this selection could totally work