Okay, if indeed it's an early 60's frame, it should be what is called a 'round chassis' here in India. Basically, the two tubes, emerging from the left and right of the swingarm pivot section of the frame, merging towards the rear single downtube (that hangs down over the gearbox from the main top spine of the frame) are welded to form a sort of a hoop and not bolted down. The newer frames, from the late sixties and go right up to the last Cast Irons made in India, have the same bolt-up system that your bike currently sports. These late sixties bikes too had a G2 prefix on their engine numbers.
Sorry to be a spoil sport, but I thought I ought to let you know.