It also came with an interesting story that appears to be correct.
This frame was given to Ken Evans (Cycling Editor) by Gerald O'Donovan. It appears to be one of two frames built by the SBDU to use at Harrogate in 1977. This snippet from an article about the show has an SBDU frame in the foreground and an SBDU bike in the background. I believe that mine is the built up bike at the back as the front frame has a standard rear brake bridge. I think they built this frame to be light; everything about this frame is light, they made the frame detail more interesting than normal SBDU.
The transfer layout of these frames is slightly different to standard Time Trial Special frames. The seat tube of the frame in the article has two Red/Black/Yellow TI-Raleigh coloured bands above and below the seat tube badge, with the Ilkeston oval at the base of the seat tube instead of the chain stays - identical to SB1500. The frame number of SB1500 also comes from this period of 1977. You can just about make out that Ilkeston oval on the bike at the back of the picture. If you squint you can also make out a single drilled chain ring. The fork column of SB1500 is stamped with 'SH' or 'HS' depending on how you hold the fork.
After Ken passed away, this and other frames he had such as a Hand built Carlton TT frame were put in the loft - that is where a slight bit of moisture has got to some areas of the paint.