As the song and all the related chords center about the key of Bb, I would think without a doubt they are referring to an open fifth Bb chord, that is, Bb and F notes only.

One of the problems with trying to represent chords in a font without superscript and music font capabilities is shown here..... How do you express, on a normal BBS, the difference between Bb5 (B-flat open five chord) and Bb5 (B major flat five)?

Some places use the convention of separating the chord type from the chord note, thus; Bb(5) [or sometimes just Bb5] and B(b5) which offers far less confusion. Perhaps it is one we can adopt here?