Bolt size

It appears that this is the exact hardware a handyman used to hang a door in my house. Problem is, the bolt does not screw in securely to the trolley assembly. Once the door is hung, the bolt slips, the door hits the floor and the system is useless.

The bolt thread and assembly thread seem in good shape. An M6 thread seems to hold securely, but I cannot find one with the slight enlargement of the shank near the bolt head to secure it to the door bracket (keyhole like cut).

So I wonder what thread size on bolt of this is in 23-193 package. I really only need the bolts but would buy whole kit if I was sure the bolts would work.