Just resurrecting this thread to say thanks for the pics. I've just bought SB1988 which is currently masquerading as something completely different and these pics gave me the confidence that it is a SBDU frame. In due course they'll enable me to put it back to near original configuration.
In the shed:
Raleigh Team Pro - SB1988
Ribble 531C - "The Real Steel Special" - custom paint and Shimano stuff
GT Hardtail - Y2K frame plus hand-me-down parts
Specialized Transition Expert - modern aero TT rocket