The plug insert looks remarkably well made. It looks to have been turned in a rather precise, if crude, lathe (look at the the detail of the groove). This is even more impressive if you think that the first real modern lathes only date from the late 18th century. The plug insert could've been moulded by some sort of near net shape method, but I doubt it (the top part of the plug seems to be much more rougher in nature). The pics are too low resolution to make a definitive judgement.