Wouldn't surprise me if its just a di from the distorted amp sound mixed back.  Check this out  https://www.youtube.com/watch?v=Uc3SxgT4uCw

@SonicExplorer

I think what your refering to is the side effect of the preamp/OD boost.

I found a good example of this today in the intro to Joe Satriani's "Crushing Day"
 

Maybe a bit, to me that song sounds more natural though, could be simply chorus and some EQ, along with widening.    The stuff I'm referring to is not just chorus and EQ, there's something else happening, you can tell by the un-natural frequency carvings and harmonics in the upper registers.  Back in the day, typically the drums and bass were tracked (sometimes with scratch guitars) and then guitars rigs were carefully mic'ed to accommodate the song, with tweaks to the EQ on tracks coming in.  Then in mix the guitars were already 90% there, they usually needed just another small nudge here or there on the track EQ.  Granted, yes, there were things such as an outboard parametric EQ or a 31 band EQ, but those were usually not used for guitars.  Point being, today with a DAW we can do all kinds of frequency carving, notching, etc - many many bands.  This entire approach was typically NOT utilized back in the day, rather as I explained.  So while today it is theoretically possible to get an EQ to help create the sounds I'm referring to, it is definitely not what is heard on the recordings I'm referring to. Especially given the same thing is heard on different releases with different artists and producers.  No way is that some fancy, complex analog EQ carving coincidence.   And not just attributed to reverb or modulus effect, or multi-track widening, rather something additional is being utilized.   Once your ears latch onto it, you'll hear it in various songs from the early 80's.   And then you can't ever un-hear it.  

"...un-natural frequency carvings and harmonics in the upper registers..."
That perfectly describes a flanger. 
 
And yes, it was all over the place in the 80's. Its popularity began in the late 60's, but back then it was used at extreme settings as a special effect. Think "Itchycoo Park".
 

 
Back then, the effect was not easy to achieve, requiring two tape recorders running in parallel and manually slowing one of them.
 
Then along came the Electric Mistress stompbox, which took it from being a tricky studio technique to something anybody could use. More importantly, it gave users more precise control over the parameters. That, and the fact that it was cheap, made it a standard piece of gear in the 80's. By then guitarists had figured out that if you made it a little more subtle and had it follow distortion, it fattened up the guitar sound without sounding like an obvious gimmick.
 
 
Here's an example of more subtle use of flanging in 1977:
 

 
It does that by filtration. Specifically, comb filtering. A comb filter creates a series of narrow, harmonically-related notches that are most audible in the upper frequencies. When static, it sounds like the inside of a pipe. But when the filters' frequencies are modulated, the continually changing filters create complexity in the spectral distribution. And if your signal is already spectrally dense, e.g. distorted, you get that thick interesting sound.

Hi. A slow flanger/phaser, maybe?

JR