My opinion (very strong, I might add- but, it's just an opinion..) is that, YES, done probably by all of the factories or front-runners. Often tried and never caught or it didn't matter on back markers... 2. some was "uintentional".... case in point; ( Car Life, Wagging the Dog article).... " Ford had only produced about 700 Boss 302's at the start of the season... considering the SCCA could not find a single Tunnel Port Mustang the year before.... obviously, Ford never intended to sell TP's, but sold Boss cars quite rapidly..... parts homologation/production is another story.... then we get into tech, specs, weights, etc by individuals...... My opinion? Sure, a ton of these stories are part of the fascination and lore of the times,cars,and people we idolize..... the "good 'ole days"....... Let me pose this question to the few who actually competed at the time; At that time, did it make you really mad when "they can't do that?" .. " I can't buy that!"..... "You can see as plain as day that's not right!" came out of your mouth? I can tell you as a current vintage racer, it's probably more prevalent, and USED to get me very upset..... I build and race my car LEGAL and CORRECT, even if no one else does..... Would the racing been better had no one 'cheated'?? probably not- just a tad slower, with a few slightly different results in a few instances.... but again... just my opinion.. Ken