To me, that's the first indicator that a guitar might be post 1980'ish. You'll see a lot of people claiming that they have, or are selling, 70's vintage Alvarez guitars, but have sound-hole truss-rod setup.
I have another Alvarez from 1981-2, that adjusts in the sound-hole. As you can see from the response from SLM, the headstock truss-rod adjustment was on the earliest Alvarez guitars. FWIW: I have the same guitar, and it is around the same vintage as yours, with a 4 digit serial number and the headstock truss-rod adjustment.