As far as I'm concerned a repro that's using backwards compatible parts is perfectly accurate. Sometimes repro builders get a little too hung up on what something is by the book rather than what it became in the field.

For the sling, yes mounting the front on the FSB is something you can see a lot in Vietnam. M16s were designed with all the sling points facing down, which reflects the existing doctrine of the time and the earlier doctrines that considered slings something used for marching and transport, but that rifles should be entirely unslung in combat. The M16s and Commando carbines (with the later carbine having their buttstock slingpoint facing up but the forward one down) were light enough that people started carrying them slung in combat, and figuring out that mounting slings on FSBs worked better.

At some point, which I unfortunately can't pinpoint, the doctrine flipped to follow what people in the field had figured out and the modern U.S. military teaches that rifles absolutely should be slung in combat. You can see in early Iraq soldiers and marines still attaching slings to FSBs, as well as a ton of using up or side facing Velcro mounting points to achieve a similar outcome.

Edit: Here's 80s and 90s Delta doing the same thing, giving a clearer view of the refined version of the technique.