Hi rkhairajani,
The bike in your photo has Gold Star type rear mudguard stays, The RGS should have arrow head stays
The speedo/rev counter mounts are also more like Gold Star than RGS
The RGS frame is unique to the model, it has several different features to the A7 numbered frames
More close up photos of the bike would be needed to offer further opinions
These frames have been copied and faked by replica builders
I would not contemplate buying an RGS without a dating letter /birth cert from the Gold Star owners or BSA clubs
Known history and old documentation is further provenance of an original bike
Regards
John