Thank you for the prompt reply, MarioD. After further investigation I've discovered that REAPER's MIDI display is offset by one octave, so where I thought key #76 was F5 is in fact F4, at least according to https://computermusicresource.com/midikeys.html, i.e., there are two MIDI conventions one octave apart based upon where they locate middle C. Who knew?

So, in answer to my question, the two mystery MIDI drum notes generated by BIAB are Hi and Low Block respectively. Problem solved. smile