The block for your jib you are referring to I believe is for your spinnaker halyard. Both jib and main halyards path into the masthead sheave box and ride over two sheaves, one forward and one aft. The sheaves are ganged together. You can see the pairs of sheaves here (this masthead is not original but similar to the original B29 unit). This is the forward side of the masthead.
The main halyard goes up and over the two starboard sheaves aft to fore, out the forward end of the masthead and down to the main winch and cleat on the starboard side of the mast. The jib halyard goes thru fore to aft on the port two sheaves and down the mast to the port winch and cleat. The main halyard is always cleated on the starboard side of the mast.
The B29 does not have internal halyards.
The original halyards were wire with rope tails.
Your hull number should be stamped into the builder's plaque at the aft side of the cockpit. The hull number should also be the number on your mainsail.