Does anyone know how to connect the outer spool assembly to the trimmer shaft on a Bolens BL100 weed trimmer?

From looking at the parts (in the attachment) and the part breakdown diagram, I do not see any logical way to re-attach the outer spool. The "stem" on top of the outer spool has a square inset so that it will fit inside of the shaft and connect to the flex drive shaft. However, there is nothing to hold the parts together. Any ideas?

Thanks!