®i©ardo wrote in news:3m0oj.15304$3m6.4485
@fe2.news.blueyonder.co.uk:

Indrek Aavisto wrote:
The third tail fin is unusual. Any ideas what it was for, and why it was
dispensed with on most models?

Cheers,

Indrek Aavisto


The first conversion from Avro Manchester to Lancaster made use of about
75 per cent of the Manchester's parts and assemblies - and the
Manchester had the additional central fin - hence this is probably that
aircraft, serial number BT308. Note that it lacks mid-upper and ventral
turrets.

The second prototype Lancaster, serial number DG595, flew in May 1941
and incorporated mid-upper and ventral (feature of the B Mk1) turrets
plus a newly designed and enlarged twin tail configuration which
discarded the central fin.



Since there was only one "upper" turret why was
it referred to as the MID upper turret?