Helicopter pilot noticed improper installation of Tail Rotor Weight Plate Bolts during pre-flight. Aircraft removed from service.

Narrative

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. A spare aircraft (identical model) was flown in to cover the base - Aircraft Y. I flew Aircraft Y on two revenue flights and one cancelled scene call. Aircraft X was returned to service and returned to base by me [a few days later]. I completed one revenue flight. The night pilot; on preflight; noticed the two tail rotor weight plate bolts were installed backwards; on both aircraft. Maintenance was consulted; and both aircraft were removed from service. Both aircraft are in the process of correcting the discrepancy and being vibration checked before being returned to service.Ensure AMT's and pilots are made aware of the correct orientation of the through bolts; that the 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.