In 1915 Hotchkiss, a French manufacturer of machine guns built a factory in Coventry because the advancing Germans were about to bring their operations in France to a halt.
After the war, they turned their attentions to engine making for William Morris among others, culminating with him buying the place, lock, stock and metric thread dies in 1923. He could have converted everything to BSW/BSF but chose not to probably because of the expense.
A lot of the automatic thread cutting machinery would have been geared specifically for metric, thus all Morris, Wolseley and of course MG engines and transmissions used these old French metric threads up until 1955.
They are a fine metric thread of 8mm x 1.0mm pitch and are used in all places on the XPAG engine and gearbox instead of 5/16" BSF.