IIRC India had contracted with Sterling for a large shipment of conversion kits to convert No.4 rifles to 7.62 NATO, then some sort of tiff developed over Sterling not being authorized to sell the Sterling SMG to India so India canceled the order.
I suspect most of the sterling No,4 conversion kits that hit the surplus market came from that canceled order.
India had used the SMLE style rifles almost exclusively but they had a stockpile of No.4 rifles and these were used by their Navy.
Pakistan manufactured the No.4 rifle under license, but I have not heard of any Paki No.4s being converted to 7.62.
PS
I don't think India was illegible for SEATO but was illegible for NATO membership. A number of NATO members are nations you might think would have been part of SEATO.
Its likely they chose to stay out of NATO due to political implications of their on going confrontations with Red China. If India had been part of NATO the border skirmishes with Red China could have triggered WW3.