Eurocopter Captain reported finding the tail rotor retaining bolts installed incorrectly during a routine inspection. Subsequent inspections of the other Company aircraft found one other aircraft with the same incorrect installation.

Narrative

On DATE; the laminated half-shell bearings were replaced on Aircraft X. The aircraft was ground run for vibration checks and returned to service. Several flights were conducted until the aircraft went out of service for an unrelated issue on DATE1. A spare aircraft (same model) was flown in to cover the base- Aircraft Y. I did the ground runs on Aircraft X and had 6 subsequent flights before the aircraft had an unrelated issue DATE1 and was taken out of service. I did not fly the spare aircraft as the unscheduled maintenance issue on Aircraft X happened at the end of my last shift for that hitch. I had two flights DATE2. I also had two flights DATE3. One flight on DATE4 and the aircraft had an unrelated issue DATE1 on the 6th flight. I was told by NAME lead that another pilot found the two tail rotor weight plate bolts were installed backwards on both aircraft. Maintenance was consulted and both were removed from service. [I suggest to] ensure AMTs and pilots are made aware of the correct orientation of the through bolts: that fasteners are to be exposed and visible. Perhaps a note or caution in the maintenance documentation as to the importance (if any) of correct orientation of the bolts.