Hello and welcome Sean, Unfortunately late T-Series serials overlapped early C-Series serials, confusing things for us Hi Power fans. The T-serials continued long after the T-258 you cited. For instance, T-374117 is shown in Blake Steven's book. I have seen photos of others significantly higher than the T-258xxx, or T-261xxx commonly cited. I suspect your gun is approx. '69 - '70 production.