August 1960, the very first batch of B-15s, number 178, which could mean it was the 78th.  This is actually an A-15, "A" for accordian, little known fact that the these early B-15s were originally designed with stereo inputs to support the two sides of the accordian.  Unique double baffle, 5U4 rectifier, this is not the loudest B-15 but it's the best sounding one.  What is the grille cloth made from?  Whatever it is , it's beautiful.  All in all an extraordinary piece of bass amp history, super, super rare!   Speaker is non-original, requires a stepdown to operate in the UK.