Below is the main shaft. The shaft has a reemed section part way up which is where the mast collar goes
above the top mast bearing. I'll highlight this in a lter photograph but for now note that this reemed section
is where the collar goes.

The mast inserts only one way up into the rotor head. The instructions denote that the mast has two holes
drilled at either end, the 4mm hole (smaller one) should be at the bottom. The following show the mast inserted
and the swashplate linkages connected.