Free Airmail To Saigon But With 5 Precancel Stamps Attached

Hi All,
I have this rather confusing cover with 5 precancel stamps attached yet it looks to be a free post for military. Do you think this is probably because it was being sent to a collector of stamps / covers at that time?
Any help always appreciated.
Cheers,
Jeff

The unusual range of stamps (dates of issue) makes me very suspect.
Don

If this originated in the USA the military free postage would be invalid. If it were originating in a foreign country other than SE Asia & destinating in South Vietnam I'm not sure. If the unit assigned stationed in a foreign country was not in direct support of the military efforts, I'm not sure I was of the belief free military was valid in combat /support zones back to USA. Would like to see a USPS/USPO regulation from manual.

The range of precancels instills fear as well. Plus what does 25 cents pay in that period of airmail? Nothing correctly.

Ah interesting it could be a fake - I hadn't thought of that. I am just interested in old letters / stamps with a lot to learn of course. Appreciate you taking the time to reply. I bought hundreds of letters in an auction and many are very interesting and I try and do as much research online as I can before posting the odd one here.
I will post any others of interest if I find them.
Cheers

Assuming it is legitimate, my initial and quick assessment would be: The free frank would work within the USPOD/USPS and U.S. military postal systems. To mail outside the U.S. system would require postage, in this case by using an airmail-bordered envelope, 25 cents was needed to countries in Asia, a common rate used throughout the 1960s up to July 1, 1971. APO 96243 was based out of Saigon in the last half of the 1960s and into 1973 with some related detachments. The precancels do exactly pay the 25 cent rate, but span 3 definitive series over several decades and 3 cities of origin, but were probably enough to fool the Vietnam postal workers along with the paid auxiliary mark. Thus a local Saigon to Saigon cover between two postal systems with previously-used stamps to avoid real postage payment, and not requiring a plane. The cover's date would be between Jan 1, 1965 (inception of APO 96243) and June 30, 1971 (end of 25 cent rate) and the serviceman's info could narrow this range significantly.

On the flip side, assuming it is fakery: Easy to do with the lack of postmarks and just one auxiliary mark. One could make a near-identical cover fairly easily today.

The army changed from using service numbers to using social security numbers 1 July, 1969. So probably dates to before that time.

So if we take those stamps off could this cover have been delivered? Or would there need to be other auxiliary markings? Or the fact that it says postage paid would suffice?

A search of the APO revealed that it corresponds with Phu Lam and the 1st Signal Brigade (an element of the US Army Communications Agency) in what would have been South Vietnam. It's about 190km due west of Ho Chi Minh City (old Saigon).