What exactly is the Purāṇa Saṃhitā? Some say it was created by Vyāsa as a blueprint for all Purāṇas that came later.
Do any Purāṇas discuss what's it about? Do the terms Purāṇa Saṃhitā and Mūlasaṃhitā mean the same?
There's one Sanskrit version available on archive.org. Is this the same that was authored by Vyāsa? Are there any English translations of the same?
It is a "reconstructed" text according to S. P. L. Narasimhaswami. The summary to the introduction to this "restored" samhita is available here (From the Journal of Sri Venkatesvara Oriental Institute).
In my opinion though, it is the work of academic ignorance attempting to find a reason to disqualify something it does not understand as fiction. The author dismisses, even the slightest abstract or esoteric, of the majority of puranic literature as it exists today as false mytholgy invented by people. It looks like he even creates his own version of the puranic concept of the four Yugas. I would not incorporate this text into any serious research.
According to tradition, the original amount of verses comprising the Puranic lore is 100 crore (100 x 10000000). For source see Shiva Purana:
1.2.57-58: “The Purāṇic lore at the time of the first creation as conceived by Śiva contained a thousand million (hundred crores) verses. In the Kṛta age Dvaipāyana and others condensed it into four hundred thousand verses which in the beginning of Dvāpara age was separated into eighteen different Purāṇas.” (source)
As well as this page which mentions the following sources for the above statement: Brahmāṇḍa-purāṇa I.1.39-40,173; II.21.9,37; 28,96; 35.63,88; III.19.23; 42.31; Matsya-purāṇa 3.3; 53.3-4,9; Vāyu-purāṇa 1.11,60; 9.69.
