HST▸ (High Speed Train) was definitely a 'last minute' decision caused by acute set shortages mainly caused by the fatality near 'Tuckers' crossing, Bradford on Avon. Set was operated by a 'High Speed' (HSS▸ (High Speed Services)) crew (Bristol based presumably), as they had the traction knowledge, with them being piloted by a 'West' crew (Bristol or Westbury based), as they had the route knowledge.
One way an untrained eye can tell if an FGW▸ (First Great Western) HST set is formed of SSL▸ (Short Swing Link bogies (125)) fitted vehicles is that the letters SSL or LSL‡(resolve) are usually displayed at the ends of the carriages, depending on which type is fitted.