Graeme,
Do you think these machines could predate the DT models? Dick Sulway's bike looks to have the sloping top tube of the TT model, so could be a stripped down 'TT' - hence the brake 'disc' but no caliper. There is only the mounting bracket for it, between the fork tubes. Paddy Dean's bike also looks a bit special - it appears to have an extra tube bracing the rear triangle on the drive side. I guess, at this stage, all sorts of experiments were taking place to get the right balance between handling and flexibility! Often improvements were only persieved - if you thought it handled better, then you rode faster!
Regards,
Eddie.