The short of this is, Moss Motors has not been able to supply drain valves of this type that do not leak. Their "solution" is to provide a hidden internal plug to stop the leak, but which in turn defeats the function of the drain cock. You then have a plug that looks like a drain cock, which may be of interest to some concours enthusiasts, but you then have to remove the dysfunctional part to drain the coolant. At least they are honest about this and have posted a note in the catalog.

To express my humble opinion, as a career machine design engineer, this is a cop out, a sort of a ploy to allow them to continue to sell the defective parts. Of course the factory originally produced parts that did not leak, so it is definitely not impossible. But in this case I will choose not to tell them (or their suppliers) how to do it (because they have often ignored my advice in the past).