"The exceptions I mention have two filters, one for upper sideband and the
other for lower sideband, and the BFO is the common point between them:

------- -------
| LSB | | USB |
| | | |
| | | |
|
|
BFO
goes
here.

In this case, it works the same, but since there are two filters
the BFO stays on the same frequency while the filters are switched.
Not too common, since it's costly to have that extra filter."

The SX-101a has two crystal controlled oscillators (selectable one at
a time for upper or lower sideband) for mixing the first IF signal
down to the second IF frequency. The BFO is deactivated during AM
mode. So what I am saying is that even at the widest selectivity
setting, switching between sidebands on AM mode requires retuning, as
if the two crystals have too wide a gap between them. I am going to
try and check them, but the output is probably way too low for my
counter (needs 100mv min).