I agree, Formula C with the rotary induction. Could be from a bit earlier than that, Greg.
I ran TM’s, and by the mid-90′s, most of the head designs were going to a more rounded shape, like the TM KV95, compared to the previous generation like the TM KV92 that had more of a square head shape, like we see in the picture.
That said, all I remember from the pre-KV95 days was TM and Pavesi. Once the KV95 came out, there seemed to be a lot more options – Vortex, CRS, KZH… Could be that they came into the game with an older head design? I don’t remember anymore.
That could even be a real old reed valve. Look up the TM KL. It’s the predecessor to the K7 reed, which was the first shifter I had (and still have one in the garage). The KL was a reed with intake on the side of the case.