The usrp_oscope.py and usrp_fft.py code always selects subdevice (0,0) if no subdevice is specified, while it claims to:
"If there's a daughterboard on A, select A.
If there's a daughterboard on B, select B.
Otherwise, select A."

The reason is that the pick_subdevice() function is broken:

def pick_subdevice(u):
"""
The user didn't specify a subdevice on the command line.
If there's a daughterboard on A, select A.
If there's a daughterboard on B, select B.
Otherwise, select A.
"""
if u.db(0, 0).dbid() >= 0: # dbid is < 0 if there's no d'board or a problem
return (0, 0)
if u.db(0, 0).dbid() >= 0:
return (1, 0)
return (0, 0)

The second if should be:
if u.db(1, 0).dbid() >= 0:

This patch fixes this for both files