"However, I ended up with a handful of once-fired Magtech .38 Special "C" (for "corto"?) brass which appears to be the diameter of the .38 special casing but the length of the .38 S&W casing."
Magtech may call it a .38 Special C (Colt??). but it sounds very much like the old .38 Short Colt, which is still being loaded. Dimensionally it is much the same as the .38 S&W Special, except the case length is approximately that of the .38 S&W. It's an old-timer, but is still in use for some types of competitive revolver shooting (ICORE) in which .38 Special revolvers are used, as the cases will be ejected completely, whereas, .38 S&W Special cases will not. That speeds reloading. It also can be used in revolvers chambered for the .38 S&W, but it is slightly undersized for that purpose. I have read that some have found that the .38 S&W cartridge can be chambered in .38 Special revolvers. While that may be true, I can chamber the .38 S&W in none of my .38 Special/.357 Magnum revolvers. I've tried just to see if it can be done.
"A friend of mine has a S&W 38 short and it says
Property of US Army on the top of the sight groove. The only numbers I can find for model number is 35290. The gun has a 4 inch barrel and it is in very good shape too."
Your description does not add up for a Victory or pre-Victory. First, a 4" barrel would indicate either a .38 S&W Special caliber (except for a very few early pre-Victories chambered in .38 S&W for South Africa), or that the barrel has been shortened from the 5" standard. The proper caliber will be stamped on the barrel. Second, a Victory model would never have a "Property of U. S. Army" topstrap stamping. The only stampings ever used on them were "UNITED STATES PROPERTY," "U. S . PROPERTY," or "U. S. NAVY". The only "Legal" serial number will be the one on the butt, and will probably have a "V" prefix.