Prospector, that 'constriction' is the end of the chamber proper. The smaller diameter area forward is the 'throat' (at least from where I hail, possibly other terms in other parts of the world). That is NOT the gas ring. That is what keeps .357 Magnum ammunition from being chambered. At least when everything works correctly.
If one measures that forward shoulder, the measurement should match up with the length of an empty case from the cartridge in question (in your instance, a .38 Special).
The gas ring is a very short 'tube' arrangement on the front of the cylinder, surrounding the extractor rod. The purpose was to keep the burnt powder gases from firing from entering the fissure between the extractor rod and the cylinder itself. The burnt gas is usually full of fine grit and accelerates wear.
IKE, I'd believe someone rechambered that revolver for .357 Magnum at some point. It was more common with stainless steel revolvers, but not unknown in the blue steel guns. Quite often, they would not accept some of the heavier bullet loads as the bullet would project out the front of the chamber.
I'm pretty adventurous at times, but I don't believe I'd run much .357 Magnum ammo through that revolver. In fact, I would look the cylinder over with a magnifying glass (after a good cleaning) looking for tiny cracks or other evidence of impending failure prior to shooting it.