Jadunath Sarkar's Shivaji and His Times (1919) provides some background to caste enmity through the historical figure of Shivaji. Although we do not know for certain that Dr. Ambedkar read Sarkar's book, the work elicited such popular and scholarly interest and controversy that by 1928 it had already run into its third edition; it is therefore likely that Dr. Ambedkar came across it at some point.
The following passage describes Shivaji's conversion into a Kshatriya before his coronation:
"...one great defect had to be removed before [Shivaji's] coronation could take place. He had to be publicly purified and "made a Kshatriya." On 28th May [1674] he performed penance for his ancestors' and his own sin of omission in not having observed the Kshatriya rites so long, and was invested by Ganga Bhatta with the sacred thread, the distinctive badge of the twice-born castes like the 'pure' Kshatriyas of Northern India. The next step was to teach him the mantra (sacred verses) and initiate him into the rules of the Kshatriya caste. Shivaji very logically demanded that all the Vedic verses appropriate to the initiation and coronation of a true Hindu king should be chanted in his hearing, because the Kshatriyas being one of the holy 'twice-born' castes, he as an admitted Kshatriya was entitled to use the Vedic mantras equally with the Brahmans. At this there was a mutiny among the assembled Brahmans, who asserted that there was no true Kshatriya in the modern age and that the Brahmans were the only twice-born caste now surviving! Even Ganga Bhatta was cowed by the general opposition and evidently dropped the Vedic chant and initiated the Rajah only in a modified form of the life of the twice-born, instead of putting him on a par with the Brahmans in this respect. This purification and its sequel, the investiture with the sacred thread, were performed with 'great ceremony'; a vast amount of money was distributed among the Brahmans, Ganga Bhatta alone getting 7,000 hun and the crowd 17,000."
Sarkar, Jadunath. Shivaji and His Times. Calcutta: MC Sarkar & Sons, 1919, 271-73.
Annotation by: Koirala/Pritchett