From the control layout, I'd hazard a guess that it might be either a 68 or 69. My grandparents had one (different cabinet, though), that set was still running as late as 1988 or 1989 with minimal repairs when my brother dumped it (he took over my parents farm when my dad passed away, and he thought that there might be too much radiation coming from an old set for his kids. IIRC, the only major issue with that set was that the picture always had a greenish tinge, even when new. But as I say, it was reliable.