Here is an update to my "Is this a first flight cover" that initiated this thread.
I put the cover out on eBay
and it sold for a miraculous $102.50. The winning bidder was kind enough to explain the cover to me when I inquired as to its provenance.
Although the cover is postmarked July 29, 1938, the successful bidder is convinced that the year date is incorrect. He, and at least one other bidder (there were five different bidders total – most dropped out just below the $50 mark), were/are convinced that the correct year is 1939. He explained to me that the envelope itself was not available in 1938, but was created for the 1939 transatlantic service by Imperial Airways.
He believes the cover in question was meant for the Southampton-New York flight, but for some reason missed the flight. He said the cover was posted with enough time allowed that it should have made the flight, but did not. He pointed out that prior to the summer of 1939 there were no regular flights across the Atlantic. The flight mentioned in my eBay
write-up (the same flight suggested by the above SCF participant in this thread) carried only British newspapers as an experiment and did not carry any general mail.
Finally, he stated that his collection revolving around these 1939 flights totals in excess of 450 covers and that the cover in question is only the second one that he's seen franked with Russian stamps.
For those who have read this thread and care, I hope this provides some closure (but maybe even more questions?).