The category 'erstwhile princely states' was applicable:

From 1947 to 1949, princely states acceded to and integrated with the Indian Union through Instruments of Accession and Merger Agreements; in this process, a number of smaller princely territories were absorbed directly into the neighbouring former British provinces. When the Constitution came into force on 26 January 1950, its First Schedule formally classified the remaining territories into four categories: Part A states (the former Governors' provinces, now including those merged smaller princely territories), Part B states (former princely states and unions of princely states, each headed by a Rajpramukh), Part C states (former Chief Commissioners' provinces and a few smaller retained princely states), and Part D (the Andaman and Nicobar Islands). This four-part scheme was replaced by the twin 1956 enactments — the States Reorganisation Act and the Constitution (Seventh Amendment) Act — which reorganised the country into linguistic states and abolished the Part A/B/C/D distinction.

Applying this to the item: the category the question names, the 'erstwhile princely states', refers to this constitutional classification — chiefly the Part B states, with a smaller number as Part C — which existed as a distinct, formally recognised administrative class from the 1950 Constitution until the twin 1956 enactments dissolved it. Taking the whole span together, from accession/integration (1947-48) through the 1956 reorganisation, this class of territories was consistently treated apart from the ordinary former-British provinces; this is the sense in which the category was 'recognised as administrative units' through this period.

• Confining the category to “around 1947-48 only” treats accession/integration and constitutional classification as the same moment, when the classification into Part B/Part C units was a separate, later (1950) constitutional step that remained in force until the twin 1956 enactments.

• Placing it “post-1956” reverses the sequence: the twin 1956 enactments are precisely when this classification was dissolved, not when it began or continued.

• Denying that any such category existed overlooks that these territories were explicitly given distinct constitutional categories — Part B, and in some cases Part C — in the First Schedule from 1950 onward.

Hence the category was applicable between 1947-48 (accession/integration) and 1956 (the twin Reorganisation/Seventh Amendment enactments), when these territories were treated as a distinct administrative class.