The six Convention states are Chamba, Faridkot, Gwalior, Jind, Nabha, Patiala. In my experience in general this area is of least interest to Indian collectors. To collect British India stamps with overprints is complex and more interesting to some than without. Overprints and surcharges add opportunity to find many error, some of which are very scarce or rare. Coupled with the fact there is a surprising variance between states, makes this area of collecting a rewarding one
The India Study Circle (ISC)
Sun Stamp Co.