If the belt groove is 1/2" wide, it is '55-'56.  If it is 3/8", it is '57 or later.  The three allen screws have been installed by someone to repair a slipped damper, or to prevent it from slipping.  It probably won't function as a vibration damper with the screws in it.  The marks have been added to allow timing to be set, also the 90, 180, 270 degree marks were probably used to position the crankswhaft for valve adjustment.

John