When generating filenames to save messages b4 am looks like it truncates
the message ID to generate the default filename (or possibly uses a
portion of the date now I think about it, I didn't read the code).  For
example

   https://lore.kernel.org/r/20200320232515.GA24800@embeddedor.com

results in a mailbox

   20200320_gustavo_embeddedor_com.mbx

which is all well and good except if you also try to save:

   https://lore.kernel.org/r/20200320232556.GA24989@embeddedor.com

it generates the same filename and silently overwrites the first
mailbox.  Using b4 mbox avoids this as it generates files using the full
message ID such as

   20200320232515.GA24800@embeddedor.com.t.mbx-thread

which avoids collisions.

FWIW my workflow is currently to save a bunch of mailboxes into a
directory then throw them at a script to do some testing & apply them if
that works.