The nut to lock the carriage to the bed is used frequently, especially when doing tailstock drilling, keeping track of and retrieving it can be time consuming.  I had already replaced the missing square headed locking bolt with a regular one and I took one of the wrenches I had bought to add a dedicated wrench for the tailstock, and used it for the same purpose.  I cut the open end section off.  I put the box end on the nut, placed so that when parallel to the bed the carriage was free to move and when it was pulled toward the operator and about perpendicular the the bed, the carriage was locked.  I center drilled and tapped the head of the bolt for a 6-32 screw.  Then I put a couple of washers (3/8" and 1/2") over the wrench and secured the parts with the screw.  Figure 1 shows the new assembly.  For better appearance, in the near future I will turn a dedicated washer to replace the ones used now.