A couple of weeks ago I found a winter mitt in some rocks by Llyn y Cŵn (between Glyder Fawr and Y Garn). It had presumably been there for some months but still seemed in reasonable condition and usable. Unhelpfully, I can't remember the make or model (possibly Marmot?), but it was a serious piece of kit, about a foot long, something like those at this web page: http://marmot.com/products/expedition_mitt
I handed it in to the warden at Ogwen who can be contacted on 01248 602080 or [email protected].