You're very welcome.
This explains the holes drilled in the corners of the bedplate - - Spring Motors didn't have them, but Triumphs did. The on/off lever is the later style, so I'll change my diagnosis to this being a complete Model A Triumph mechanism lifted out of its original cabinet and dropped into the earlier cabinet of a Spring Motor or Concert.
I should have asked for a photo of the data plate FIRST!
The serial number may also explain the machine's present state. Triumphs between roughly 45,000 and 50,000 were equipped with Model A mechanisms mounted in the later raised-panel cabinet we associate with the Model B/C/D/E/F. A collector in days gone by may have thought he had a mismatch of mechanism and cabinet, so he may put the mechanism in the earlier Spring Motor/Concert cabinet.
George P.