Just rebuilt one a few months ago. They were a direct replacement for the Q-J on many GM cars. #6528 fit 68-70 350". 68-71 400,430,455". #6774 fit 1972 350 &455". They take all the standard Holley parts. Not sure on how long they were produced, but I would guess in to the late 70's. They were advertised as the "emission
design/street performance" carb. I guess that would cover most the bases. Only draw back is they just didn't have the cfm of the QJ or TQ of that era. On the plus, lots of parts available for tuning.