It is a Cinelli 1R stem, first version. Noted by the CINELLI typography on the front plastic cover disc.
It came out earlier than 1977 though. Presented in late 1975, some availability in 1976. 26.4 bars exclusively in my book.

The wedge bolt should be well lubricated on the threads and at the wedge face.
These stems really liked the then current production bars (last of the shield logo) as the knurling at the center was deeper than the earlier bars. We lubricated the bars too, but there was some argument about that.
This stem came supplied with an aluminum quill bolt too, be sure to lube the brass washer(s) at the base of the bolt head.

That all written, I did not like them. Too fussy. I have two now, one on a bike as that was original equipment, the other for reasons I cannot recall.