The most significant design change for the M8 was the movement of the magazine release from the European preferred "heel" of the pistol to the rear of the trigger area, where it is preferred by American shooters.
The M8 debuted in 1981 with the very first prototypes pulled from PSP serial numbers 40163-40165. This was followed by another series of further prototypes with serial numbers 001 M8 to 0044 M8. A final production of 10 units from 10001 M8 to 10010 M8 readied the series for finalization. Series production commenced with serial number 81251 and continues to this day. Helmut Welde oversaw the development of this variant as with all P7s. Four of the prototype models in the serial number range from 10001 to 10010 were shipped to HK USA on February 4, 1983. These pistols were sent to the New Jersey State Police on March 4 for tests. The NJSP eventually adopted the P7M8 department wide, and it continues, despite an abortive attempt at switching to the Smith and Wesson SW99. On February 7, 1984, the guns were returned to HK USA, where they were then sold to "Powder Horn Antiques." From there they disappeared into American collections. (This information from Heckler & Koch, The Official History)
Additional changes to the M8 series were a thicker trigger, improved final design heat shield and enlarged trigger guard. The thicker trigger has most recently been reverted back to the normal thinner trigger on current M8s.