I took some time off this thread in order to gather more evidence for my theory by doing extra runs. Then, having a decent amount in hand, I could calculate the statistical values as Fabian had suggested.
However, now the whole concept is a subject to change. My recent findings show that the accepted belief that the LK chests are UNIQUE in terms of 16-bit programming is false. In fact, Infernal Pit and Pit of Acheron special chests, that were postulated to have 2\32 = 4.3 billion combinations, also possess the same behavior as LK chests! Moreover, I suggest that ALL chests in D2 including “sparkling” and “super-special” perform a similar model.
Out of 45 total screenshots with HRs I’ve done in both PoA and IP SC areas I found 4 (four) pairs of the same patterns. Obviously, that is improbable for 4 billion combinations, as it was presumed. Actually, even one pair is enough for a proof – that is just not possible to happen
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Here it is:
1. Pul rune, PoA SC, p7
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2. Pul rune, IP SC, p7
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3. Gul rune, IP SC, p7
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4. Gul rune, IP SC, p7
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The main conclusion is that we have a finite amount of patterns (presumably 2\16 = 65,536) for the Infernal Pit (or PoA) Special Chest. Thus, by finding all patterns, as it was done for LK, we can see the whole picture of this place in terms of efficiency for rune hunting. For example, if there would be at least another one Zod pattern – the Infernal Pit should give better chances for finding Zod in game (in general, 1 out of 5170 for a dropped rune), as I have suggested in the title of the OP. By now we already have screenshots of one pattern for Zod and two for Cham.
Other mini-conclusions:
1. PoA and IP chests have different patterns (until proven otherwise). All my experience (around 8K runs in both areas) shows that IP SC is much more profitable than the PoA one.
2. Unque, set, rare, and magic qualities are decided by another mechanism after the pattern is chosen (analogously to LK magic/rare).
3. The quality of an item is preset in the pattern.
4. Etheral/socketed quality is preset in the pattern (one example – bardiche).
Hope, we will eventually find the truth!