Well, the Howatt part seems to be unanimous. I am not a Howatt expert and do not have any of their catalogs, but I have a couple of guesses.
By the way, how long is the bow?
First, I have been told that Howatt employees sometimes made bows for themselves that were like the "production" models, but did not have any decals (I do not know about other markings). If this is true, then I would not be surprised if one of these employees made himself an "upgraded" model. Anyway, this may be one of those bows.
Second, as far as the "initials" HM go, I have seen both an older Howatt Mamba Hunter with a serial number that started with HM, and a Howatt Hunter NE Plus Ultra of similar vintage to this one that had a serial number starting with HM.
So, here are my guesses. I am looking forward to having a true Howatt expert educate the rest of us.
1. Since I cannot get a good look at the finish, I can not tell whether or not the bow was refinished. But, I suspect that it has not and that it is one of those Howatt employee bows.
2. If this bow were a 62" (AMO) bow, then, because of the fancy overlays, I would guess it to be a Howatt Monterey (some of the Montereys seems to be upscaled Hunters). This is my "best guess", since the Monterey is the only "shorter" Howatt that I have seen with overlays as fancy as these.
3. If this bow were a 60" (AMO) bow, then I would guess it to be a Diablo, maybe even an "upscaled" one.
4. If this bow were 58" (AMO), then I would put even more weight on it being and employee custom job, since I have never seen/heard of a 58" bow with these kinds of overlays.
Okay, guys, there is my 2-cents. Now, let the experts step up and set us straight!