The link on The U.S. Militaria Forum (1st link below) was about a picture of 2 Marines reportedly on Russell Island 1945 with D tip slings, pictured in the book The M1 Carbine.
Pictures from Dirt Detective shown here:
Close up:
To which Mr Ricca replied:
"The photo showing Marines with D tips, it is possible the Navy produced a small amount, or altered some Reising slings."
Mr Ricca then went on to say (2nd link below):
"I am working on a post war sling modification story of the Air Force changing standard slings by shortening them, for the M1A1Folding Stock. Circa 1950's to early 1960's.
I have them in my possession but will not reveal anything until the research is done."
I do not recall him revealing anything else about that research.
If he had, It would have made headlines in the Collector world.
I have a number of different 44-45 dated Navy Cargo and Misc Field pack bags with D tips.
All I own have a Square buckle with off set middle bar.
The Links:
M1 Carbine sling - original? - SLINGS [REF] - U.S. Militaria Forum
https://www.milsurps.com/showthread.php?t=15737