When you transport the solder spool holder there is a good possibility that if tipped it the spool axle will drop out of the holder. This print creates a keeper insert that closes the slot and prevents the spool axle form falling out. Two versions are provided one is for the Hako model 611-1 and the other is for the Chinese knock off of a similar unit. The Open SCAD source is provided if you unit is not exactly the same.