Hi Richard, very observant of you to find these bell size differences! I do not have this one but all my later 250s and 955s of this period seem to have the same ca. 8-9 mm bells (the apparently universal job no. 10152 for all of them). Unfortunately no drawings for this Airport Fire Tender seem to be known, so we may never know what this diameter difference originates from. Kind regard, Jan 

Salut Richard.

Once more you have spoted some unlisted variations. Congratulations.

I supose that the tool for the bell which had been made in 1936 was worn and that Meccano had to make a new tool.