I am in due receipt of your letter Feb. 17, 1968, and have noted the contents. The sentence in question is, as you stated, Swayam Bhuva Manu and his wife, Satarupa (not Satsvarupa).
The reaction that the hunter would have suffered, had he not received the Mercy of Narada Muni, is hellish life. There are different kinds of hellish life for different kinds of sinful activities. So, one has to undergo such punishment if he doesn't take to Krishna Consciousness. It is only the devotee who can save the living entities from falling down into hellish conditions, and by the Grace of Narada Muni, the hunter was awakened to Krishna Consciousness and was saved. Yes, the general principle is that one is sent to the particular type of hell, and when he is practiced to suffer the hellish condition, he is given a similar body as reaction. These hell planets are described in the 5th Canto of Srimad-Bhagavatam; there is no question of imagination. One who is particular about this may read the different descriptions of hells in the 5th Canto of Srimad-Bhagavatam.