The Rs-series per BS 21 is an obsolete thread series and should not be used in new designs; however; you may find a requirement for Rs-series screw threads on an older product/part design drawing.
The G-series and the Rs-series British Standard Screw Threads have much in common:
a. Both are British Standard Pipe Parallel.
b. Both have the Whitworth 55° included thread angle form with Full Form rounded root and crest of thread.
c. In all situations the G-series can be used when the Rs-series is specified, but the reverse is not true.