Helicopter Mechanic reported incorrect installation of the main rotor mast was discovered by another mechanic and subsequently corrected.

Narrative

On Day 0; during my previous set of days on; (we work XX days on; X off at Company); my help was requested from my fellow mechanics in the ZZZ hangar to assemble a mast during heavy maintenance. I was on duty for the ZZZ1 base at the time. After completing my daily duties in ZZZ1; I went down to ZZZ. I assembled the mast with the team down there; documented as item number XX on page X of the XXXX continuation sheet for Log page XXXXXXX. I also signed the mechanic block on the RII checklist for item XX.X; 'Main Rotor Mast - Assembly only.' The assembly of the AS350 Main Rotor Mast is performed with the assembly upside down; because that's what the manual calls out as there's a planetary gear assembly that lives inside the conical housing (at the bottom of the mast) that is installed during mast assembly. Once the assembly was completed; we turned the Mast Assembly right side up; secured in a stand and went home for the day. I needed to be back in ZZZ1 and off the clock with time to spare (our working day is limited to 12 hours) so that I could be available for my 24 hour on call responsibility in ZZZ1. I clocked just shy of 9 hours that day.Fast forward to today; I learned that the ZZZ2 mechanic discovered that the bolt that holds the rotating scissors drive collar in place was installed backwards. It is supposed to be installed in the direction of rotation. I see how this can easily happen; considering it's installed while the Mast is upside down. The ZZZ2 Mechanic took the aircraft out of service and corrected this discrepancy upon discovery. Suggestions - 2 things:1. Pay more attention to the nuances of the figures in the AMM. Complacency definitely played a role here. I've assembled many of these masts; and the direction of this bolt isn't called out specifically in the text. It is however illustrated in figure 1-2. 2. After flipping the mast over; I should have checked the assembly for correctness from top to bottom. Instead; I went back to ZZZ1 to keep time available for my on call responsibility. This was my only involvement for this maintenance event. Maybe I would have caught the backwards bolt had I been involved in the installation of the mast assembly. Maybe not. Impossible to know. Regardless; I messed up by not double checking my work for correctness at the end of the Mast Assembly process.