Well....I did a quick google search for melanopus marshall islands...and found this photo shot in the Marshall Islands by none other than Dustin Dorton (President of ORA) -
http://www.dustindorton.com/diving/marshalls/wauday1/pages/melanopus.htm - very clearly a fish with black ventrals.
More digging showed more fish in Marshall Islands cited as melanopus:
http://www.dijitalimaj.com/alamyDetail.aspx?img=%7BABAB2194-B843-4D04-A467-6BAFBE8EE0D7%7D http://fishbase.sinica.edu.tw/photos/thumbnailssummary.php?ID=4654 - note the photos from Marshall Islands...all have black pelvic fins.
So this leads me to two ideas. #1. If these fish are from the Marshall Islands, they'd have to be Frenatus, if Frenatus is in fact found sympatrically with Melanopus in the Marshall Islands. Melanopus photos from the Marshall Islands show classic black ventrals, which are one of the known indicators of the species.
That said, as one reference puts it, geography is the best way to distinguish Frenatus from Melanopus...which implies that they're not generally found coexisting in the same geographical regions. I cannot confirm that frenatus doesn't exist in the Marshalls, but indicators suggest that it shouldn't, because Melanopus is there. To me, that more likely means that #2. regardless of what species it is, it's not likely that it's from the Marshall Islands (since it would appear that the tomato-complex species found in the Marshall Islands is in fact A. melanopus, and clearly distinguishable as such)...and as such, I'd think it's a tomato (Frenatus) of unknown provenance.
Some additional possibilities - they may have been Tomato clownfish (A. frenatus) that were shipped out of the Marshall Islands, but did not ORIGINATE (aka. were not collected) in the Marshall Islands. Another possibility, they may be frenatus/melanopus hybrids...although if you know these were wild that's not a likely scenario.
I gotta convey that for a LONG time, I myself didn't know the difference between Melanopus and Frenatus...I certainly didn't really know when I was 16 or 18 years old an working retail back in the early 90's. I have a feeling it was a similar scenario to the way that "Sebae" was (and still is) a label applied to both true sebae and dark forms of Clarkii, or the way that Ocellaris and Percula still get confused. I'm still learning that discerning species often requires some very subtle cues (my latest revelation was how to discern Betta splendens, imbellis, mahachaiensis, smaragdina, and smaragdina "Guitar"...a matter of tiny dots and stripes, facial pattern differences, and not much else at times).
I will add this - it's categorically safe to say that the darkness of the body is the one thing that ISN'T diagnostic, and is highly variable, particular in A. frenatus. Seems to typically be that males and young fish are more likely to retain the clean red flanks, while females and older fish darken up.