I think those are Guide 682 C lights, and all the ones with turn signals on top are starting to get desirable. I would adjust them perfectly, and then put a very small tack weld on the nut and threads or nut and mount (if it is steel) That way you could always grind away the spot weld if you ever need to remove them, but a thief wouldn't have the luxury of that much time.

I drove my '27 daily for years, and the only things that were constantly stolen were the dice off of my tire valve stems, and one time my fire extinquisher. Usually it was kids on bikes who stole the dice. Little Urchins.

Don