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Why would one supposed letter [corrupt,mind you] exist and yet no LXX[72]?

That is the question: the false letter or at least the same information of the false letter is at attested to as existing around 80 A.D. by Josephus.
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Just how old is the real first instance of this letter?

I don't know, but the fact that similar information as to that false letter is given at length by Josephus indicates that an LXX translation must have existed for Josephus to be talking about it.
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Plus I do understand that this letter is not to be trusted.

The letter dubious, but that does not mean that the LXX did not exist B.C.
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Y trust tainted materials as the Letter and the LXX[72].200AD.

Why trust the exaggerated Maccebee books as an indication of Jewish history in the 400 silent years? Why trust Church Fathers historical information that has been preserved by Roman Catholics? I do not "trust" the LXX, just say that it is reasonable that it exists, and that it has (extremely) limited uses. The date is 275 B.C. for the beginning of the LXX.
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Very interesting that he did not site one verse as an example,and that he had to do some of his own hand at translating because there was no copies for him to use.Hmm.

I have only read some Josephus, and I have not come across this.
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Even you accept that the letter and the supposed complete LXX[72 in common use of that day are dubious at best.

The letter is dubious, but attesting to something. The LXX exists, but is far from perfect. You have to separate the existence of the LXX from the existence and nature of the letter.
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So Y the support? I still don't get that.

There is no support other than of what appears to be reasonable facts. Agreeing with the KJB translators that the LXX existed is not the same as upholding the LXX as modernists do.

It is like believing that the Vulgate was at the basis of Wycliffe's Middle English Bible. I don't support the Vulgate, but the fact of its existence or influence cannot be denied.

Doing verse comparisons between the Hebrew, LXX and New Testament (all presented in English) is not a sufficient way to prove or disprove whether or not the NT followed the LXX. English translations can be twisted to say anything. If someone takes the OT reference from the KJB, the NT quotation of it from the KJB, I would have to rely on King James Bible supporting people like Edward Hills or Dr Thomas Holland to give their opinion as to where the KJB is following the LXX. Since I believe that Christ has provided for the Church different ministries, I would have to accept that these two men have been sent of God to show the facts in this matter.

Accepting the historical existence of the LXX, and understanding how the KJB translators used it is the most sound approach. It is interesting that Dean Burgon said in 1881 that acquaintance with the LXX should be a requisite for anyone editing the KJB, which was fulfilled in Redpath, famed co-editor of the Concordance of the Septuagint. Its interesting, but of the Septuagintical translations I have seen, Ezra 2:26 has something like Gaba (or Gibeah) rather than Geba. I am saying that other than knowledge of the Septuagint, and the Hebrew and the Greek languages, there was no deliberate correlation made between the LXX and the making of the Pure Cambridge Edition.