Both calibers are headspacing off the rim on the nub at the top of the chamber. Sorry for the technical terms, im not well versed in 1911s.
FYI, the "nub" on the barrel as you called it is generally called the "hood".
Up until the 1980's to 1990's, 38 Super barrels were chambered to head space off the rim. It was one of the reason 38 Supers had lack luster accuracy. Of course, there were only a couple of manufacturers making 38 Super guns at the time.
In the 1980's, competition shooters got interested in the 38 Super due to the greater magazine capacity and better ease of making major category. They found if they chambered the barrel to head space off the case mouth, accuracy improved greatly. Eventually, the manufacturers followed suit and just about every new barrel made these days for a 38 Super will head space on the case mouth.
Just a bit of trivia.
The barrel that came with my modified 38 Super 1911 has a sleeved chamber. 38 Super ammunition would not chamber.
The 38 Super case is just a bit larger in diameter down near the base of the case than a 38 Special case.
Since your barrel chambers 38 Super ammunition, I suspect your barrel's chamber was just lengthened to accommodate the 38 Special ammunition.
If your barrel has the same external dimensions of a standard 45 ACP barrel, it would be fine and safe for shooting 38 Super with the appropriate recoil spring. But accuracy will suffer.
I'd get a new barrel, bushing and recoil spring fitted to the gun, keep the 38 Special barrel and have fun with both cartridges in a 1911.