OK,I couldn't find any of my door lock pictures pertaining to the rear barn door.I went out and pulled the lock that is in my suburbans barn door.I took some pictures of it for you.
Basically you need to grind the end of the shaft to a triangular shape.The lock shaft is 1 3/4" to 1 13/16" long and the length of the bevel on the end is 5/8" long.I would cut the shaft to length,then grind the bevel at the tip or end of shaft.The size of the square portion of the stick are the same as the front doors stick.Pay attention to the position of the bevel verses the lock,this is important.
door lock pic#1 door lock pic#2 door lock pic#3 door lock pic#4 door lock pic#5 I hope the pictures help.
Tim