In reply to birdman: A sighting compsss is a general term for a compass used to take/set a bearing at eye level. The Silva type 15 (or its modern equivalent-I'm out of touch) uses a mirror with a hairline that allows you to read your bearing off a reflection of the bezel, or to set a bearing by seeing the needle align correctly; look over the mirror to see your target.
A prismatic compass is a more traditionally shaped compass with a flip-up sight and, opposite it, a prism. The prism creates a "split screen" effect so you see the target through the sight and, just below, a view of the compass bezel through the prism.
Time was when you could buy ex-army ones at two bob for a bucketful - one in good nick would be a lovely piece of brass workmanship. Sit down for the next bit - I saw ex-army marching compasses for sale at a surplus depot in Ripley last week at £250-odd apiece, and they weren't new. Your friend had better be pretty special!