Your assumption is false as even the FBI said that no one but LHO was listed to receive mail at his Dallas P.O. Box.
Do you have a cite for that? Harry Holmes, Chief Inspector of the US Postal Service for Dallas, stated that the Part 3 of the post office box application form contained a list of other people authorized to receive mail at that box and that Part 3 was thrown away (in accordance with post office practice) when the box was closed in May 1963. So, the fact is that we do not know who else may have been listed to receive mail in Box 2915, Dallas in March 1963. Here is his testimony (7 H 527):
MR. LIEBELER -- "So there is no way, as I understand it, to tell from the records maintained, as far as you know anyway, who was authorized to receive mail at Post Office Box 2915 that Oswald had while he was here in Dallas before he went to New Orleans in April of 1963; is that correct?"
MR. HOLMES -- "Other than Oswald himself and his name on the application."
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