Back in the day, insurance could get quite spendy for drivers who had some extra tickets. You could order Challengers/Barracudas without the "S" in the vin, thus lower insurance, but still able to order performance without all the brass band stuff. I have a JS29N car, has the max cooling, A33 axle package, flat hood, but still has the HP 383 for the engine. Has 383 Four Barrel on the fenders. Obviously, 440's/426's were not on the menu. Believe they were even available with 4 speeds, but don't quote me on that.
They were also know as insurance cheaters/beaters.
Actually every insurance company I dealt with back in the 70's didn't give a crap about the S in the VIN, the wanted the fifth digit & if the engine was supposed to be 350 CI or bigger you paid more... 340 were insurance beaters, 383-2 paid the big premium without the extra performance....
Had a buddy with a 71 Challenger R/T Curious Yellow, black stripes, wing.. The car screamed "LOOK AT ME" but it only had a 340.... Of course we all know 340's ran, but as far as the insurance was concerned it didn't cost any more to insure than a 318 Challenger....
383-4 with a flat hood? Thats someone who's not telling you what he's got.... Yes, a "Sleeper" but there was no factory package called a sleeper, it was just a bare bones model with the biggest engine you could order short of a flashy car...