<HTML> Marcel & Del,    Del,  there was a Mason "A"  that had three extra one half size ribs in the high treble.  The last one that I replaced the board in I took out the small ribs and replaced them with standard size and number of ribs. It did not seem to make any difference.  It was a player and I think that it  was  an  'A- 4'.      I have an A-3  #21158 in my shop now.  It has the standard number and size ribs.      Marcel,  If you don't have much experience in replacing  soundboards it may serve you well to practice on a less valuable piano.  Also let us know witch "A"  this one is.  You can find this number in the left back of the plate.  Regards to All, Jack Wyatt </HTML>