What is the serial number? That would hold a clue. The bottom bracket area appears to be welded and unfinished. The seat stays appear to be brazed which is how I think Chris did them when Gary Helfrich was getting started with TIG / Plasma welding around '84. When I showed up near the end of '86, Gary had already switched back to conventional TIG. While I mean no disrespect, Gary's steel TIG welds were not the smoothest I've ever seen. Also, by then, Gary was committed to titanium and striking out on his own with Kestrel (later Merlin) so his head was not in the same place.
After I got things going, I discovered how to use a pulsed current technique that yielded a significantly smoother weld and the bead width could be held to about +/- .010" (0.3 mm). That also helped making the heat input much more consistent which gives manual TIG a big advantage over manual fillet brazing. We also were able to drastically reduce our finishing time since the welds now required very little filing or sanding.
The reasons brazed fillets are finished are appearance and reduction of abrupt changes in direction and cross-sectional area. That helps the fatigue life of the frame which is more important for mountain bikes that tend to be ridden much more harshly.
So, my guess is that the bike is an '84 with a TIG welded main triangle and brazed seatstays.
All the best
Scott