I had one of these mounted on an old Rudge three speed. I put in the thousands of miles that you would expect for a teenager who had no other form of transportation, and lots of time on his hands. I am sure that my downhill runs, (of which there were many), went well over 20 mph. I put downturned bars on that bike too, because I couldn't afford a real lightweight road bike. (I also removed the fenders, and the chainguard, convincing my parents that I was entering a period of delinquency.) The added aero advantage, plus youthful enthusiasm, convinces me that I must have also spent a good time on the flats going over 19 mph. (I have better equipment now, but I'm almost four decades older, and presently I spend almost no time below the dreaded 19 mph cut off.)
All this leads me to believe that the precaution to not exceed 19 mph is similar to my departed aunt's precaution, not to eat raw mushrooms. Nothing bad happened. The wheel didn't lock up or self destruct, and I was hit by no lighting bolts. I wouldn't worry. However the Lucas produces a ticking that I consider decidedly uncool. If you have to go retro, I'd get a circa 1981 Avocet cycloputer, or just do what I do now, which is to ask my riding buddies how fast I'm going.
Garth Libre in Miami Fl.