Okay, I'm confused. The only difference between a 38 spl case and 357 case is the 357 case is 1/8" longer, so it won't fit in a 38 Spl cylinder. Assuming you have not resized the neck, a fired 38spl/357 mag internal lip diameter (ID) would be around .357 (the bullet diameter for both 357 and 38 spl) which jives with what you said However, if the case is necked down to .357, that would seem to mean the outside diameter (OD) below the shoulder is larger than a 38 spl/ 357 mag case, yet it fits in your 38 spl. When you say, "It also is exactly at the length of a 38 special", are you talking about the entire case or the length to the shoulder. You may mean that it is a 357 mag case with a reduced case mouth so it would fit inside the headspace stop of a 38 spl, but that would require the bullet to be smaller than 357 as the stop inside the cylinder has internal dia of 357.
OTOH if the case mouth of a 38spl is reduced to 30 cal, then it is a 30-357 (aka 30/357 Paxton), or less likely the 300 Max Cat, which is a necked down (.30) 357 max case . Both utilize .30 rifle dies to neck down the case. Check it out; https://dandtcustomgunworks.websitetoolbox.com/post/300-max-cat-30357-5744675