Elgin introduced their Lord Elgin line of wrist watches in 1937. These were 21j watches, in solid gold and gold filled cases, and even platinum. The had the streamline, Art Deco style of the times, like this one, my latest acquisition.
While the 531 had a split balance and blue steel hairspring, the 556 had a solid balance and white alloy 'Elginium' hairspring - presumably similar to Hamilton's 'Elinvar', and the plates and bridges were beveled. Elgin still damaskeened the plates of the 556.
Late in the 680's life, around 1955, the 688 was introduced, not to supercede the 680, but as a complement.
The 688 was essentially identical to the 680, but instead of the fixed balance wheel cap jewels, it had Incabloc shock protection. Elgin had introduced shock protection in their 641 series in 1950, but it was not until 1954 that they made a shock protected Lord Elgin.
This was also the last of the 8/0 movements, which had been introduced 20 years before with the 519. Millions of 8/0s were produced, many of them for the military in WWII. But I don't collect military watches, so someone else will have to blog about those!