Take a look at the third reference in the Bayer/McCreight paper from 1972 -

3. Landauer, W. I.: The balanced tree and its utilization in information retreival.

IEEE Trans. on electronic Computers, Vol. EC-12, No. 6, December 1963.

The term was coined before Bayer ever used it, even though NIST provides conflicting information - http://www.itl.nist.gov/div897/sqg/dads/HTML/btree.html