F.a.m. 5 And F.a.m. 8 Combination Cover With Postage Due And Po Seal

I was gifted this cover by a friend because he knows I collect US Postal Seal covers. He mentioned that it is a less common airmail cover because it is a 'combination two F.A.M.s on one cover'. He also mentioned that he thought that the cover had originally been franked with only 20 cents but that it was returned postage due and the sender added the third 10 cent stamp.

Once I looked closer at the cover, it appears to me that was originally franked with 30 cents and at St. Louis the St. Louis PO had some confusion as to what to do with it and what the franking should be. They mistakenly applied the rubber Postage Due stamp, opened the letter, and then a day or two later sent it along re-sealed (with both RUSH and PO Seal) to Brownsville for the F.A.M. 8 flight.

Anyone who might agree/disagree or have other opinions on how this cover travelled through the postal system would be greatly appreciated.
Don

Seems plausible. The cancellation overlay backs up your thoughts that all three stamps were on the original letter. The non-filled in Return for Additional Postage backs up your thoughts on confused post office personnel. I'm not an "Official Seal" collector, but I am assuming that these were only applied by Post Office personnel after examination of contents ... not by accidentally ripped open items. Awaiting other thoughts.

it looks like all 3 stamps were applied at the same time as the cancelation is on all 3. If the 3rd stamp was applied later then the cancel would not line up as shown.