Just out of artistic curiosity- in terms of traditional depictions, would you say it’s more likely that the brisingamen was closer to a closed-front necklace or an open-front torc?
Given my background, my first move is to always look toward the word itself -- if men means one thing specifically, then we know it's that. Unfortunately, so as far as I can tell, the word men ('necklace,' basically, but with this bit of ambiguity apparently) doesn't really differentiate between different types of neck jewelry.
I can show support that it was not one of the strings of beads and pendants that women would wear suspended from the front of their dresses attaching at brooches -- one of those is called a sǫrvi, and Snorri actually gives us that definition confirming it was current in his time: En fyrir því er kona kennd til gimsteina eða glersteina, at þat var í forneskju kvinnabúnaðr, er kallat var steinasörvi, er þær höfðu á hálsi sér. 'And for this reason is a woman referred to in reference to gemstones and glass beads, that there was in old times a woman's accessory, which was called a stone-sǫrvi, which they had on their necks.'
It is very possible I'm missing something here, since this is a semantic context I'm not really used to, but usually when the word men is used in Old Norse, there aren't any context clues that tells us what the men in question looks like. There is one instance where the men has a tygill, a tie, like something to tie it onto the person's neck, so it's definitely inclusive of something that goes all the way around, but that wasn't really in question anyway, and this particular men is on an idol of the Finnic Jumala (Jómali in the Old Icelandic) so it might not be the best example to extrapolate from. There's a men in Vápnfirðinga saga that seems to be somewhat more solid (the footnote calls it a hálshringur, a collar, but in-text it is just called men), but this also seems to go all the way around and have a way to tighten it.
I can't find a separate word for a torc, and even in Modern Icelandic I can't find a way to say torc without describing a type of men (or other word for neck jewelry).
When we look to archaeology we see that there's a bunch of stuff that fits sort of this somewhere-in-the-middle description, stuff that's pretty solid but does wrap all the way around like this rather torc-like neck-ring from the Hiddensee hoard:
Or these, from the Høn hoard. I'm not sure, since I can't see it especially clearly in the picture, and I'm not an expert on this stuff, but it looks to me like the back parts of these could constitute a tygill like the men had that was fastened to Jumala in the aforementioned saga description:
And going back a bit to the Migration Age (when we might imagine that stories about Brísingamen already existed and were being shaped into how they would later be told) we find incredible stuff like this:
Here's a gullgubbe with a necklace, looks like it's maybe got some big chunky stones on it...
...or perhaps they're not stones at all, but seeds. Be sure to check out this discourse about drift seeds and Brísingamen (also known as 'sea-kidney'). The papers referenced are here (Meaney, paywalled but you can get it on sci-hub) and here (Tolley -- this is giving me an error trying to load, but it downloads and opens fine).
Finally, either the single most important piece of evidence, or not at all, or somewhere in the middle (lol) is this:
This is a silver pendant found in a grave in Aska, Sweden. It's usually accepted as a depiction of Freyja. Personally, I like to exercise a little caution when identifying figures that get found with little secondary context so I'm gonna stop short of saying that it is Freyja. But there's also no real reason to reject that hypothesis, either. The figure here definitely seems to be wearing something around her neck with a prominent big round thing in front (seed? jewel?). It's not exactly the clearest depiction, but it might be an actual depiction of Brísingamen (which means the person who owned it wore a necklace on their necklace).
Anyway if the sea-kidney stuff is true, then it seems reasonable to me that there could be a tradition of describing Brísingamen in terms of that, making it a sort of special case in comparison to other jewelry and fashion. If it's not relevant here, then I guess the average Scandinavian would just picture whatever the coolest neck jewelry they could come up with would be, and that may well be one of those sorta-solid neck pieces that still attach in the back. Gonna tag @thorsvinur for backup here, he knows much more about what people actually wore and thought was cool in the Viking age.
