I have found that sling, I guess, but I don't remember where. It has a different icon and indeed does not report the increase in Thac0 in its description, but when I access the PC's report the +3 is present.There is no sling in DSotSC or NTotSC. Not sure where you got it but if a monster in DS or NT had the item, they had a standard item in the game, not something that comes with either mod.
Alex
i don't exactly remember where i found it but i've been under the impression that it was in an area near the forest of the forgotten souls, an area that doesn't exist in vanilla BG1