Electrostatic concept plz help?

No, there isn't any field lines between sides R and N.

Just imagine before the block RN is inserted into the space between the parallel plates M and S, there is field lines produced by the charges on the plates. The direction of which is from M to S (i.e. from left to right).

Now, when the conducting block RN is put into position, there are equal but opposite charges induced respectively on sides R and N. These induced charges tend to produced electric field lines in direction from R to N (i.e. from right to left). These induced field lines will superimpose on the original field lines produced by plates MS. The two fields then cancel one another, leaving no field line exists inside the block.

It is generally true that inside a conductor, there must be no field lines. It is because of the reason that induced charges occur in a conductor, and these induced charges will produce an "opposing field" to the "applying field", producing a "no field situation" inside the conductor.