You have a duplicated quad on the non sloped outside surface. Also the top part of the underside area is missing. (20 years ago, 14-Jun-04, to lugnet.cad.dat.parts)
(...) of the underside area is missing. (...) SNIP 2003-03 New Parts Preview (URL) 30249.dat - Slope Brick 55 1 x 6 x 5 (URL) w. (20 years ago, 15-Jun-04, to lugnet.cad.dat.parts, FTX)