Though they don't actually use the term, the tracking device sounds like it's based on the principle of a laser, so maybe one of the first, if not the first depiction of one in film or TV.
The car Steed drives is a 1937 Alvis Speed 25 Tourer.
The original script has Steed enter the shop after Franks threatens to kill Linda, but it was dropped and we cut straight to Cathy and Graham in the lab.
Steed is in a hurry to chase after Rankin but checks himself and says, "Sorry, after you, ladies first", ushering Maggie and Cathy before him. This is completely ad lib, it's not in the script at all. Steed's last words in the scene according to the script are "I think I know where..."