Can any one illustrate to me the manuscript evidence supporting "the great day of their wrath" as opposed to "His wrath”?
First of all this is not a contest between the text of Erasmus (TR, ς) and the modern critical text. The manuscript evidence for the plural and singular forms of the pronoun is very evenly divided. The committee for the third edition of the UBS-GNT assigned the adopted reading (αυτων plural) a grade of C (A-D grades) which isn’t a great deal of confidence.
The argument really hinges on which reading is the easier. The easier reading is considered secondary based on the probability of a correction by the scribe to make the text more intelligible. Bruce Metzger judged the singular reading more intelligible since it agrees with the nearest antecedent “the Lamb” who is understood as the agent of wrath. David Aune (Rev. WBC, vol 2, p. 386) considered the singular the more difficult reading since two agents of wrath are mentioned in the the preceding context; “The one seated on the throne and … The Lamb.” Arguments based on the difficulty of the original reading and probability of change to clarify the text are in my opinion not very convincing with this particular variation unit. Both readings are intelligible given the context. Neither is particularly difficult.
The Codex Siniaticus (01) reading is always transcribed as αυτων “their wrath” but looking at the image of the manuscript it is not unambiguous. It appears to be αυι (nonsense) corrected in the column space with ων. We should bear in mind that Siniaticus is not the same class of witness for the book of Revelation that it is for the Gospels. It takes numerous liberties which you can read about in J. Hernández Scribal Habits …. Readings from Codex Alexandrinus (02) are somewhat more reliable. Due to the lack of early manuscripts the minuscules (generally byzantine, e.g, 94, 1854, 2053, 2344, 1006, 1841, 2351) play a bigger role in determining the text of Revelation.
The manuscript weighing for any contested reading in Revelation is a complex process which cannot be summarized with any hope of success. The editors of byzantine text editions (Robinson-Pierpont 2005, Hodges-Farstad 1985) have given special attention to the text of the Apocalypse which can be found in the introductions to their critical editions.
Rev. 6:17 "their"
6:17 [αυτων] 01, 94, 1854, 2053, 2344, syrph, WH, 04, 1611, it(ar), it(c), it(dem), it(div), it(gig), it(haf), it(z), vg, 1828, 2020, 2329, syr(h), Haymo, de Promissionibus, Oecumenius, Fulgentius
Rev. 6:17 "His”
[αὐτοῦ] 02, 025, 1006, 1841, 2351, copsa(ms), cop(bo), 205, 209, arm, 2138, 046, 1859, 2042, 2065, 2073, 2081, 2432, 2814, Byz, eth, Andrew, Arethas, Beatus, Primasius, ς
 Juan Hernández, Scribal Habits and Theological Influences in the Apocalypse: The Singular Readings of Sinaiticus, Alexandrinus, and Ephraemi Mohr Siebeck, 2006
Translations of Revelation 6:17 differ on the basis of whether their underlying Greek text reads ὀργῆς αὐτοῦ (‘his wrath’) or ὀργῆς αὐτῶν (‘their wrath’).
The 16th century Textus Receptus Greek New Testament on which the King James and other Reformation-era translations were based offered the former (as does the 4th century Codex Sinaiticus, though in the form of αὐτῶ).
But by 1881 Westcott and Hort had evaluated the manuscript evidence differently and offered the later -- ὀργῆς αὐτῶν (‘their wrath’) -- in their influential critical text, as do the leading contemporary critical texts of Nestle-Aland, UBS, and SBL. Most English translations since 1900 follow this change, the NIV, for example, providing 'their wrath' as the primary reading with a footnote, "Some manuscripts his." This verse is not among the earliest preserved papyri fragments of Revelation.