I was there during that time, do not recall seeing that stock. Definitely not on my rifle.

I have book somewhere with fairly good photos of GI’s on patrol around 72-73. Will see if I can find it and take a look. I would imagine they would have seen service during and after Vietnamization / withdrawal.

Very likely E Stock Late M16A1 made it in as the ARVN were still getting fresh equipment and funding until 1974 when congress decided to lose the war purposely to burn down any legacy Nixon had.  Probably not many saw use by American troops. The older XM16E1's were never withdrawn from line units once in country, they just had buffers and and barrels changed out when the better parts became available

I remember my late uncle mentioning once that they felt almost ashamed seeing ARVN units with brand new A1s while they had their very early rifles handed down from one soldier to another within their unit in country.

That ARVN troops liked colorful badges and scarfs, painted helmets and tailored fatigues is another story ...

You touched on a point not often talked about, the ARVN were defunded, I recall an article in Time or Newsweek about the drop in support for the South Vietnamese, one data point was towards the end they were only getting 200 rounds of ammo per soldier per month!

They did it again with Afghanistan- even with the same narrative that Kabul would not fall, they said in 1975 Saigon would not fall but stay free.

Update on the photos, found the book and they are from 70-71 so no E stocks, just a lot of non regulation uniforms and peace sign necklaces.

If I had an easier way of posting photos I'd share some from my collection. The Marine Corps especially kept issuing old xm16 models into the 1980s. They were less common in Army units at the end of the war but I have a couple clear photos of them being used by infantry in Cambodia 1970. The Vietnam/Afghanistan parallels are an interesting topic. The Vietnamese in my opinion made better allies and were able to hold there own when supported with US airpower. The Kabul govt wasn't defunded, but the corruption and misrepresention of reality could only buy them so much time the ANA grunts weren't getting paid or supplied the last year. The ARVN held out several weeks in their final stand while the ANA collapsed totally in the last week leaving the door open to Kabul