It's P33581006
Here is a pic of the G3358 spindle sleeve.
Some quick measurements...
Overall length is 140mm
The threaded section is 42mm dia with 1.5mm/turn and 26mm long.
The other end is the only part with the spline, it's 56mm long and takes a 42mm snap ring.
I still have some parts to make for work, so I can't take the head apart just yet. I'll take a look at it this weekend, though.