/A/ versus /aU/: 51 pairs

Art out
baa bough
baa'd bowed
baaing bowing
baas boughs
bra brow
bras brows
can't count
carboy cowboy
carboys cowboys
card cowed
Carl cowl
cars cows 
chars chows
darn down 
darned downed
darning downing
darns downs
dart doubt
darted doubted
darting doubting
darts doubts
grass grouse
grassed groused
grasses grouses
grassing grousing
ha how
lard loud
ma mow
Mars mows
mast moused
parched pouched
parches pouches
parching pouching
parted pouted
parting pouting
rah row
scars scows
spars spouse
start stout
starter stouter
tarn town
tarns towns
tart tout
tarted touted
tarting touting
tarts touts
trance trounce
trances trounces
unbarred unbowed
vase vows