This is a 14g 50mm zinc yellow plated bugle batten screw. 5mm hex drive, 10 threads per inch, self countersinking with 4 ribs, type 17 with 2/3rds thread. Batten screws are a more presentable alternative to nuts and bolts. As they are countersunk they finish flush leaving no ugly or potentially dangerous obstructions. Use a driver bit such as
these.
