I think the precedence between this rule and the previous one should be made explicit, as they clearly can both fire at once.
You probably always want this one to win (which I think is existing behaviour, from the order).  So I think the reverse conditions of this rule should be added to those of the previous
 -- i.e. add "not 'STASH' in cm.attributes or not str(cm.attributes['STASH']) in iris.fileformats._ff_cross_references.STASH_TRANS"
(what a mouthful!)

I think _ideally_ the other rules for lbfc (lines 64-109) are also a problem, and should probably be just retired in favour of the one above this  -- i.e. we should require correct units, not just the name.  But I guess this is not the time to fix that.