The seperation of Indus Identity from the rest of India is a historical process that happened after the Mauryas and was almost final by the time of the Hepthalites--> a 700 year process..I know you posted a map by Herodotus I believe..But Arrian of Indica does define India as the country with Indus as its Western border, the Mountains of Manipur as Eastern border,The Himalayas as Northern Border and the Ocean as the Southern Border...Arrian Indica was based on Megasthene's account which stems from around 300 BCE..but within 150 years of that, the situation started changing..The NorthWest started to look itself as a bit distinct from the rest of the subcontinent..This might have been the result of heavy Greek invasion and admixture post Mauryas....You can see that from Patanjali's Mahabhasya (who was evidently from Gandhara region) which comments that a black person from modern day Bihar cannot possibly be a Brahmin, sitting in the market square like a sack of black beans. Then came the Scythians, on top of that Kushanas..These were movements of tribes who were half a million to a million in size each. enough to change the phenotype of the upper strata..The Hou Honshu written at the time of Kushnas mention North West as Tianzhu and rest of India as Juandu--North West is also mentioned as Juandu in the same text----pointing to the formation of a distinct sub-identity...After the ingress of Indo-Sassanids,Indo-Parthians and Hepthalites the process was more or less complete..You see that in Mahbharata whose final redaction date to around 400 AD..The Brahmins of the east think the land of five rivers is not fit for sacrifices anymore because of loss of caste system...But more importantly a warrior traveller from the NorthWest pines for the time he can go back home so that he can again frolick around with the big bosomed big hip women of the NW...The women who reside west of the Ravi river...Evidently the women of the East,where his present residence was could not sway his heart...The Brahmins of the East were indignant at such remarks and thought the warriors of the NW have completely given themselves over to carnal passions..(Little did they know that the same essence that makes a man ready for battle is also the same essence that incites his heart towards women---->testosterone)
of course post Mauryas till 1947, there has always been furious fighting regarding the settlement of the border between Trans Indus empires and Gangetic empires....Gangetic empires wanting to establish themselves till Sialkot/Chenab...Trans-Indus empires wanting to establish till Sutlej ......that's 2,200 year old saga..seen from that perspective, Partition makes a lot of sense, which was divided right in the middle of Chenab and Sutlej
Modern genetics suggest that the caste system is around 2300 to 1900 years old...I am of the opinion that the timespan between 185 BCE and 300 CE saw a lot of invasions from the Central Asia..enough to change the cultural,phentoype,religious characteristic of most of the NW..many (not all) in the NW started to see themselves different from their erstwhile brethren to the east...Fearing loss of kinship from NW, the Brahmins in the East launched the caste system in all earnest (in order to preserve their racial phenotype as much as possible)
I know it's a controversial take ..therefore I would proceed cautiously..if there is enough interest on your side, I would treat this initial idea to a 10,000 word essay, full with references and pictures
@Indus Pakistan @Joe Shearer @Mangus Ortus Novem