If a force f (Net force) acts on the mass(Spring Balance), it will accelerate with a = f/ m. Spring balance will only show the reading if there is net force causing deflection of spring (F= K*x).
Here f = Net Force on Spring causing acceleration
F= Force that causes deflection of spring(Which always acts in pair - equal in magnitude and opposite in direction)
In this case force in either direction (which is equal in magnitude) will account for the spring deformation and thus the spring balance reading.
When L and R are unequal let L>R, Net force F=(L-R) causes acceleration of the mass. and spring balance will show the reading equal to R.
e.g. L=30KN R=20KN
Spring Balance reading = 20KN
Net force in horizantal direction = 1o KN.