Usually not. More likely a liability issue, since some LEO Only marked ammo is +P+. They don't want you using it in your 1945 slave labor produced P-38 that you store in a bucket of salt water.Could be steel core
The ISP went to the S&W 39 and 9mm in 1968. The +P loading wasn't used by the ISP until about 1982 or so with the Federal BPLE.Think the whole "for law enforcment only" thing got started back in the 1970's with the old Treasury Department +p+ 110gr. JHP .38special. About the same time, Illinois State Police had adopted the 9mm (S&W 39's at first) and wanted a load with a specific ballistics...the "Q" loading, but I forgot how it got that designation. Could consider that the first +P 9mm.