If this is the case, then why do they quote 'stall speeds' in aircraft
specs?

To first order, it's the stall speed in level flight at 1 g at a given
weight.

You can stall at the critical AOA. When you get to the critical angle
of attack, the wing is producing a certain amount of force (lift). If
you reach that AOA at a higher speed, it is producing more force,
presumably to accelerate you upwards (start a climb or arrest a
descent), sideways (when banked, in a turn), or to just hold altitude
(if overloaded).

Is 50 kts the speed at which you would be unable to maintain
LEVEL flight?

Well, sort of, and no. It's complicated by the fact that the engine is
also producing up force (you are, to some extent, hanging on the prop)
when pitched up.

To first order, it's the speed at which, in unaccelerated flight (you
can be descending at a constant velocity), if you pitch up further, you
will start to stall, because you are already pitched up as far as you
can be without stalling.

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
for Email, make the obvious change in the address.