Okay, we're goin' in.
The Harvard Dictionary of Music defines a trill as:
"A musical ornamentation consisting of the rapid alteration of a given
note and the diatonic second above it"
Converse defines it the same way (pg 19 of the Analyitcal)
So, there is only one conclusion. I really think he wanted a rapid
striking on two adjacent strings, and not the "hammer-pull" type. The
placing of a "B" on the second string gives the greatest advantage to
a good fingering for a M2, right.
BUT...there is nothing to indicate that a unsion repitition of notes
won't sound good...especially here, but a trill is certainly two
pitches.
WAIT A MINUTE....we're talking about banjo music, right?? I think it
just got too deep. Play it any way it works!!