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