Gents,
 
On many of the early classic 80's metal albums there is something on the guitar tracks I'm struggling to identify.  It may be a recording technique but I lean more toward some form of outboard gear or trick.  Strange sizzle in the upper frequencies.  Tube preamp, harmonic/aural exciter?  It is found on Sabbath's Heaven and Hell, Maiden's Beast, Dios' Holy Diver, etc. 
 
You can hear it during the break here at 1:50: https://www.youtube.com/watch?v=YF3jeAPGhrY
 
Any ideas or thoughts ??
 
      Sonic

Sorry I don't hear any "sizzle" but I do hear a second part come in that's sort of like an organ but I think is just a guitar part layered in there. It could be a synth. 
 
Yes I think it is as you can hear a definite,, what ever that is when keyboard players wipe across the keys,, in the start of the solo/ 
 
Keyboardist Geoff Nicholls, who was a session keyboard player during the recording of this album,

It's all over the guitar track the entire song, but is easier to hear in that break pre-solo where just the guitars are playing.   It's the exact same thing found on a number of classic metal recordings in that early 80's era.  To hear it requires decent monitors and ears that have upper frequency recognition.  There's some kind of sizzle/hair/harmonics being added, rather prominently to the guitar tracks.  It tricks the ear into thinking it's part of the guitar tone, but it's really not.  
 
Sonic

Going through some of the TH3 presets (Full version), some of the classic rock presets have this quality.
 
What they seem to have in common is boosting the treble on the amp to the point of it sounding really tinny and nasty, but then using a cab/mic combination that then takes most - but not all - of it away. So basically the cab is acting like an imperfect low pass filter.
 

Could be, just keep in mind these recordings weren't made in an era with those kinds of tools.  This was either recording or outboard technique.  I doubt it was on the recording end as it would have then likely to have been attributed to a mic or slamming tube pre.  Given this is found on a number of different albums would indicate it wasn't a recording-side matter, but rather a common denominator in the outboard/mix side.  I'm trying to determine what it may have been so I can hunt down a VST that may create the same/similar effect.  I think it's also fair to assume it is not attributed to EQ as the carves would be complex in the upper harmonic registers and unlikely multiple albums would have the very same unique carves going on.  
 
Sonic 

It could be a noise reduction artifact which was the Bane of tape based recording in those day's. 
Especially this happened in the upper frequencies. To mask the tape hiss Dobly or DBX, as example, would mess around with processing the upper frequency where the hiss was. I totally forget how it works because there's no point in remembering anymore.   Go read up on Tape hiss and noise reduction..  It will start all of us old goons off yelling at you for wanting to bring back what we don't miss about using tape :) 
Like the guy who wanted to figure out how to add record scratching to his recording,,,, geeeze///

I should add this effect tended to come from the big studio/producers in the US and UK, in particular Martin Birch (Deep Purple, Sabbath, Maiden).  Normally I might be inclined to attribute it to him specifically, but this same effect can also be found on recordings out of LA in that same era.  However, you won't tend to hear this effect in other music forms, it is usually heard only on certain forms of 80's classic metal or hard rock.  I'm leaning toward it possibly being an Aphex Aural Exciter.  That device was in use during late 70's early 80's IIRC and it polished up the top end by adding a bunch of harmonics (unlike the BBE).   That said, it could maybe be a DBX unit applied to cut down on noise from a hissy guitar amp, but I don't recall that unit creating the type of effect heard on the tracks.
 
Anybody know how to get hold of Martin Birch?  

Don't know how to get a hold of Martin Birch, BUT...I've been using Wave's Aphex Aural Exciter plugin a lot lately, and that COULD be it.  Don't actually play any metal-ish stuff, but the effect sounds a bit like you describe and suits me for both the acoustic and electric guitar parts I do (in moderation).  Waves offers a free trial on their plugins, you may want to give it a spin...

Just sounds like a cranked Marshall Plexi or JCM with onboard (desk) compression.
A little bit of chorus/Flange/Phase effect also seems distantly present.

I've always love Tony's sound. Listen to the album Paranoid and you will hear amazing solos like in War Pigs. Dual solos panned right/left. But the Sound of his pick ups in the SG with the combination of a Marshall. It just doesn't get any better really.