A Mechanic reports he repaired an emergency exit lens cover that had a 45 degree directional arrow on a DHC-8-300 aircraft. He was later told he reinstalled an incorrect lens cover for the aircraft.

Narrative

I was walking thru a DHC-8-300; Aircraft X and noticed that the right-hand aft emergency exit lens cover was not secured on the aft side. I removed the lens and saw that the aft clip that holds it in place was missing. I ordered a new lens; [but] did not install the new lens; I just removed the aft clip; installed it into the original lens and installed the original lens cover. I signed-off on the write-up that I removed and replaced the lens cover when I didn't. I should have signed that I only replace the clip. It was later written-up by a crew member that it had the incorrect lens cover. ZZZ Line Maintenance went to another DHC-8-300; Aircraft Y and exchanged lens covers with the one on that aircraft.The aft emergency exit has two lens covers; fore and aft. I repaired the right-hand aft cover. The lens cover was pointing 45 degrees down towards the exit. I doubled checked and it was pointing in that position. I checked the (Illustrated Parts Catalog) IPC Chapter 33-22-00; Figure 25; Item 80 and it gives Part Number 10-XXX and that is what it had in the aircraft. I was working Aircraft Y eight days later; went and checked the aft emergency exit lens; the left side was pointing forward with the arrow; not with a 45 degree angle [arrow] pointing down towards the exit. The right side had an arrow pointing down 45 degrees towards the exit; with [same] Part Number 10-XXX the part I checked out for aircraft X; and what the IPC calls for. The only mistake I made was the write-up sign-off that I [had] Removed and Replaced (R/R) the lens when I didn't.A crew member wrote that the incorrect lens was in place; three days after the aircraft had routine overnight maintenance (RON). I only replaced the aft clip; checked the position of the arrows; IPC; and installed the original cover. Why the event occurred is that aircraft had the incorrect lens when it came into RON; but all the Part Numbers were checked.The lens cover was exchanged with another Aircraft Y. Now Aircraft Y; on the left aft side; has a cover [with an arrow] pointing forward with no 45 degree arrow pointing down and the right-hand aft side has a lens cover [arrow] pointing 45 degrees down towards the exit. Suggest determine if the lens covers [arrows] should point down 45 degrees; or straight forward.