Either sometime between 1976 and 1987 or else sometime between 2007 and today (from 1987 onwards, the company operated as "New Detonics" until it was shut down in 1992).
Serial numbers aren't unique to one firearm, and we don't know even know what model pistol you have (Detonics has more than one model in their line). If you call their customer service at (618) 476-3200 and can provide them with the exact model, they could probably do more for you than we could here.
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