Hi John,
Sorry for the tardiness in replying, I haven't come back to this thread for some time.
You asked about the belt width for the Spandau. I've looked high and low at any photo I could find of Spandau belts and 99% show the narrow 2-rivet "Parabellum" belt. I have no documentation or directorates explaining why it was the belt of choice so I have to speculate. I believe the origin was the use of the Parabellums on the early Fokker Eindeckers and the first use of Parabellums for rear guns on two-seaters.
(Sorry for the photo size, I can't get through my FTP program to repost a smaller sized image)

Here is a poor quality shot of an early huge 2-rivet belt being used on a Fokker Eindecker with an LMG 08.
In other words the 2-rivet belt became the "standard" for all aircraft use, front and rear guns. It would make things simpler for the armorers only having to load one type of belt. One would also reason that the bullets would extract more easily from the narrow belt and therefore assist in the cycling of the gun. Even on German airships you see the narrow belt utilized. German airships and seaplanes, like the Hansa-Brandenburg W.29 and W.33 utilize the heavier LMG 08's even though the LMG 08/15's were available. I have speculated from photographic evidence that Home Defence aircraft often utilized the heavier LMG 08's.

Here is a shot of the Fokker V.5 prototype at the Fokker factory, udergoing firing testing with Fokker in the cockpit with Platz helping out. This is a 2-rivet belt. There is a superb film of this firing test, that may be available on you-tube, I'll have a look for it.
The only photo I've seen of a fighter using the wide 3-rivet ground belt was a late war shot of a cobbled together, (using different parts from different planes as well as manufacturers), Fokker D.VII used by a flying school. In the photo the cowlings are missing around the guns and the empty belt exit down tube is missing from the guns as well...very unusual.
Speaking of the 3-rivet ground belt, didn't someone post a photo of the "MvR belt" that was donated to the Australian War Museum? This belt is a modified ground belt where someone has cut off the extended spacer/alignment tabs enabling the belt to go down the exit chute to the empty belt box.
(Again, sorry for the photo size)

Here is a photo of a piece of the reported belt fragment kept by the original scrounger/momento keeper that was later sold to famous MvR collector/biographer, Charles Donald, who later passed away and this was auctioned off not long ago.
This photo and fragment, if true, is one of the only supporting facts I've seen to give credence to the use of 3-rivet belts on WW1 German fighters, albeit in modified form. As I've stated I can not find
any photographic evidence to support the use of this modified form belt.
I have spoken with ammunition collectors in Germany and they have these "Parabellum" belts, although they appear to be rare and hard to find. I have several 2-rivet belts, but I believe them to be Vickers .303 belts. Some are marked/stamped ".303" and others are unmarked. They are very difficult to find.

Here is a photo of the trigger lock from an LMG 08/15 and you can see the 2-rivet belt in the background pretty clearly. I used this photo of one of my locks and belt to discuss the positions and workings of the lock for another thread.
Best wishes,
Dave W.