Joe McCabe sent me photos of this interesting addition to his collection. He’s rightly proud of this special Taylor wrist compass with its bottom part made totally of aluminium. Not plastic, but its a perfect fit.  It still works perfectly and has a nice patina. He has had it examined by many long established dealers and militaria collectors and all agree it is original, although they have never seen one before. It has no markings or date of manufacture on the aluminium part. It’s unlikely that after the war anybody would have gone to the bother of moulding a replacement back at any stage. Too much time, effort and expense required to do that. It looks like it’s either a very early war compass or a prototype. Later in the war all aluminium would have been set aside for aircraft production etc. If anyone has any insights to offer on this, please do.