Hi volksboy57 , thanks for your input (-:
We have to disagree, at least partially.
C5 would be May 1936, so is a possible number for a Bluebird. However, if it were C5, then the frame number would start with an A, so this would be, well, unlikely, there are some wierd things happen with Bluebird numbers at times.
E6 does work for C39240, a June 1938 frame.
I think there's also differences between 1936 and 1938 model Bluebirds, but I won't go there since there are plenty of people more qualified to discuss that on this site.
However, looking at the pictures, I remember I had difficulty in seeing the number clearly, wondering if it was E5 (May 1938), or E6 (June 1938). Looking again I still think E6, but I can see how it might be E5.
Best Regards,
Adrian