Laird, It sounds like your yellow brick is a fire brick, which were most common at the end of the 19th century to the present. They were used in fireplaces, kilns, ovens, and possibly some stoves. Fire bricks were made in a variety of shapes and sizes to accomodate there use as lining in ovens etc. The letters are probably a maker's mark or brand name. Fire bricks were commonly stamped with such names. You might try looking-up your bricks in a book by Karl Gurke called Bricks and Brickmaking: A Handbook for Historical Archaeology from the University of Idaho Press. Also, you might try to contact the International Brick Collector's Association. M. Jay Stottman Kentucky Archaeological Survey