The mainspring housing is stock colt, it is plastic. I replaced mine with a wilson combat V grip stainless housing. If I were you that would be the first thing I fix, I hate plastic!
Obviously all the stainless parts are added.
What I have seen in the mark IV's is that if the grooves in the rear of the slide are angled like yours are, then the gun comes stock with wraparounds, extended beavertail (though your beavertail is aftermarket), special hammer and I believe flared ejection port was not on the standard models. The Mark IV's with straight lines on the slide were standard and have normal grips, normal beavertail, and spur hammer.
Nice gun I would definitely hold on to it, Iv'e got a full 5" barrel Mark IV series 80 and I love it, its been unbelievably reliable, something these glock disciples can't comprehend.