I'm not saying a 3/8" npt adapter / plug won't fit because they will, but the thread in the block isn't tapered

Years ago my old GGR engine was supplied with an alloy adapter in this opening, when i rebuilt that engine i wanted to put a new one in as it had seen better days, I didn't know what size it was so I took it along to think auto in Isleworth so they could measure it and they measured that fitting as a 3/8" bsp, I would hope that a supplier and manufacturer of threaded adapters would know their stuff