Panels from Fantastic Four (1961 series) #67 (October 1967), co-plot and script by Stan Lee, co-plot and pencils by Jack Kirby, inks by Joe Sinnott, letters by Artie Simek
Those freckles (or, to use Susan Richards's technical phrase, "fly specks") are the miniaturized components of the circuits making up the Electronic Bracelet. Or, to use Tony Stark's usual technical phrase, "transistors."
Panels from "The Mandarin's Revenge!" in Tales of Suspense #54 (June 1964), script by Stan Lee, pencils and inks by Don Heck, letters by Artie Simek
Okay, everybody repeat after me:
TRANSISTORS DO NOT POWER THINGS, TONY
At least by FF #67 Stan Lee seems to have learned that lesson. He's got microscopic electronics here which need to be assembled in a certain pattern on a circuit board. It's a careful, time-intensive job which would require a really big magnifying glass (or Ant-Man), and even with Reed handling the circuits with his tiniest quantum tweezers, there's still the chance of human error. That's why Reed leaves the job of assembling the circuits into the body of the electronic wristband to his Micro-Tool Assembly Unit!
Voila! One fully-assembled device, assembled in mere seconds, by the computerized Micro-Tool Assembly Unit. It implants the circuits with absolute precision onto Reed's circuit board and the electronic wristband is ready! He'll have them for sale in the stores for Christmas! The holidays are saved!
Of course, you realize what Kirby has invented herea device that actually exists in the modern real world: a computerized PCB (printed circuit board) assembly unit!
YOU ROCK, JACK KIRBY!