Actually, everybody has a bit of the truth on this one.
Bob Smurthwaite did sell an exhaust blade as the J. Roberts "Vari-Speed" Throttle Control. It is shown on the plans for Vern Clements' Brewster Buffalo published in the July, 1960 issue of Model Airplane News. It was as described, a thin blade with an elongated teardrop opening. There was no fuel control. It is also shown on the plans for the Consolidated Dark Shark and the J. Roberts Mauler (Copyright 1960).
Bill Johnson, out of St. Louis, did manufacture the more involved system that included the fuel meter. It was very popular in the late 1960's. The St. Louis gang had things working pretty good in those days. I have a never used ST G40 with the complete Johnson set-up. (I'm at work right now ~ 7:37 AM EST ~ but will post a pic this evening).
There were a lot of home made knock-offs. I did one for my McCoy -60- Sterling Guardian. As Dave indicated, the fit was critical and some form of fuel control was also necessary. When the big bore Kavan carb arrived things got a lot better.