Hachette originally provided a plastic, injection moulded backhead with issue 16. Unfortunately, the backhead was of an incorrect type for the Flying Scotsman in 1928. The one they included was for an A3, which would have been correct post-war, once the Flying Scotsman had been rebuilt, but not for the 1928 model they are aiming for.
The insert in issue 16 says a replacement part made of metal would be included with issue 30, to fit any parts to the plastic backhead, and subsequent issues will include the instructions on how to fit them to the replacement backhead. The result of this is that I can make these four issues as “done” and just store the parts for those later issues.