Here are my thoughts on the subject, offered with the caveat that I am
probably one of the least musical people alive today.

Wurlitzer's smaller band organs didn't have many registers -- often
none -- to turn on or off certain ranks of pipes.  In the style 150
roll [and organ], rather than create a register system to bring on the
three trombone pipes when needed, they added a separate 3-note trombone
division playing the same three notes as in the 3-note bass division.

Early, small band organs were designed for use in noisy places like
skating rinks and were equipped with brass trumpets.  These trumpets
played from the counter-melody division, although they were not the
only pipes playing from that division.  But they certainly gave that
division a character of its own.  It is easy to see how Wurlitzer
tracker-bar blueprints came to label the counter-melody division as
"trumpets."  Incidentally, they quaintly labeled the principal division
of the scale "melodie."

In the Wurlitzer style of arranging, the counter-melody/trumpet
division did not actually play much counter-melody.  It more usually
played along with the melodie division in chords.  That may be another
motive for Wurlitzer's division terminology.

These principles continued and held true even when, in its larger and
later organs, Wurlitzer abandoned the use of brass trumpets in favor of
wood trumpets, giving a less-blaring sound, better suited to carousel
and dance hall use.

I'd be interested in the thoughts and comments of a real musician like
Art Reblitz on this topic.

Matthew Caulfield
Irondequoit, New York