at the back - there is a solid bit of metal with bolts. the stem for sale has had that bit drilled out (its a 2 piece clamp that tightens onto the steerer tube) it can be replaced easily, but i havent bothered. So thats why its cheap - you will need to put some work in to find the clamp.
thompson stem - take a look at this photo - http://www.socaltrailriders.org/gallery/data/1514/ThomsonStem1.jpg
at the back - there is a solid bit of metal with bolts. the stem for sale has had that bit drilled out (its a 2 piece clamp that tightens onto the steerer tube) it can be replaced easily, but i havent bothered. So thats why its cheap - you will need to put some work in to find the clamp.