‘Truss’ and ‘girder’ frames had an extra tube for added strength. This was necessary if a frame was tall, but it was also sometimes used as a gimmick. The Premier ‘Truss’ had an extra tube from seat to steering head.
The extra tube the Royal Enfield ‘Girder’ ran from the bottom of the steering head to the bottom bracket.
The Elswick ‘Truss’ had two crossed down tubes.
Cycle makers worked hard to give their top-of-the-range model some sort of extra design flourish to make it stand out from the crowd and many odd frame styles appeared over the years…