Thanks Eugene,
The pin did unscrew to reveal a 10mm bolt that fits the new head.
Eugene,
I purchased it new from the Husqvarna dealer several years ago so it should be all Husqvarna.
So you are saying that the square part that has the 7mm screw end screws off the gear box to reveal a 10mm screw end that will screw into the new head?
The Husqvarna dealer ordered the 578 44 65-01 for me.