I've made this request to the Glowhead Brotherhood and I'm sure I've made it before that but I'm d****d if I can remember where - oh the joys of being a septuagenarian! Problem is I have two TD Thermal Hoppers but one of them has an unusual NVA and I'm wondering if this is a genuine Cox variation or something my dad added. These engines are part of my late father's 1950s collection of aero-modelling kit that I inherited. Anybody out there able to enlighten me (or even tell me where else I posted this!!)