What is a millihelen ?

A milihelen is a unit of beauty

Helen of Troy in causing the Trojan War was said to have had "the face that launched a thousand ships". Therefore a face beautiful enough to launch one ship would be 1 millihelen.

We've heard of Troy weight: pounds troy, troy ounces .

Correct, Jenny. This is another unit on that system.

We, obviously, did not adopt it , years ago, as being metric and presumably the Americans never will.

When I think of ship launching of years ago our Queen Mother and Eleanor Roosevelt come to mind. That must have been under the Imperial/Customary system of weights and measures.