Melvyn Douglas's undershirt

Mmmm, it's not really the same thing, to be honest. The undergarment Douglas was wearing was made from a loose-fitting stiffer cotton (not a form-fitting "jersey" fabric) and it had a slightly elasticized waistband area (that was not too "cinchy", just gathered a bit at the waist). This style of union suit, introduced sometime in the 20s and sold well into the 50s and 60s, was traditionally known as an "athletic" union suit, probably because the legs stopped just above the knee and it had a tank top instead of sleeves (likely to allow for ease of movement, as opposed to the form-fitting, full-body traditional union suit). In any event, because it was all one-piece, I imagine it was a comfortable alternative to wearing a separate boxers-and-undershirt combo that could frequently become untucked throughout the day.

Hanes sold a version of this athletic union suit which they called "Samsonbak"; in addition to buttons up the front that one buttoned after stepping into it, there was an off-center vertical "vent" in the seat that usually closed with just a single button. Interestingly, the year before "Theodora" was released, men's "briefs" underwear were first introduced to the market. But with the advent of war just a few years later, a shortage of elastic for non-military underwear resulted in a return to the old button or tie waistbands on men's underwear (perhaps giving the athletic union suits a second life for many more years to come, and, I'm guessing for some, old habits were simply too hard to break).