Due to the way the lighting is placed in the Los Angeles Police Historical Society's North Hollywood exhibit it has so far proven very difficult to gain a acceptable picture of this rifle in its entirety; the lights cause too much glare on the exhibit case.
I would like to offer a large thank you to Robert Stoner (USMC Ret.) and Tyler Beck (USMC) for walking me through my confusion about the modification to this rifle.
The one modification that stands out is the presence of the magazine release flap located forward of the trigger guard.
This magazine release was removed so a shelf could be welded in that prevented the use of a full auto trigger pack. There exists companies that will retrofit the original H&K magazine latch without removing the shelf, the rifle is returned to pre-modification usability whilst still keeping it from being able to accept the select fire trigger pack. The rifle's paintwork looks to be uniform so this would appear to be a professional restoration, something that when compared to the Norinco rifles stands out as being a level above.
The only user influenced modification appears to be the use of extended 40 round box magazines.
The rifle was disabled toward the end of the engagement in the Bank of America north parking lot by a strike to the magazine well. The rifle suffered two additional strikes, one through the upper receiver cover and the second through the attached magazine, although these strikes would appear to have happened once Phillips had discarded the weapon into the Chevrolet's trunk.
The HK91 is a firearm originally built in West Germany, it is the civilian counterpart to its military cousin the H&K G3.
Like the Norinco Type 56 S it was built to look like its military cousin yet is restricted to operate only in semi-automatic mode.
The H&K Model 91 was originally imported into the USA as a civilian sporting rifle by two independent importers between 1962 and 1975, until Heckler & Koch took over the importation procedures themselves right up until the 7th July 1989 import ban which stopped 43 different breeds of semi automatic weapons being bought into the USA under the ruling that they were not generally suitable for sporting use.