So, I can't even find the "6mm BR Remington" SAAMI spec. Did this get deleted? Is that why they are presumed the same now?
No, but the issue began with a question on the shoulder datum length between the two, and those two have always been identical.
That is not to say there are no differences in the throats, small differences in the rims, etc., but it is saying that there are no differences in what the OP needed to shop for in terms of Go-No-Go gages. Those two use the same gages. While the older ammo prior to 1996 might not meet even the previous 1989 standard, it isn't because the two have a different shoulder datum length.
When Remington sold their first factory Remington 6mm Bench Rest case in the 70's, it was a thin walled 308 based case but it was small rifle primer and meant to be finished by the loader. Their market was BR competitors who expected to have to dimension their reamers for the bullets they used and the trend in the early times was lighter bullets than what we would recognize later on. It was a while before Remington standardized their design in 1989, but they had been selling a 40X production rifle for a while.
Later on and far away... Norma standardized on a competition round that was very close, but used a different weight range for the bullets so the biggest differences began with the throat. In 1996, the Norma 6mm BR had minor differences in the case but it used the identical Go-No-Go gages. The throats were cut to accommodate higher BC bullets used in 300 meter competition, and their pressure went up to 58740 PSI from the Rem version which was 52000 PSI.
Both the Sierra manual and the Berger manual have write-ups on the history. The Berger manual still treats them separately but the Sierra manual combines them.
In 2022, the user must focus more on the throat and neck for the brass and bullet they plan to run, but the label on the Go-No-Go gage doesn't matter since they are both the same standard length.
If you go back a few posts and read the story that RusselJ linked in post #5, it pretty well covers the history of both as well as the ammo.