The BMW R100GS PD was built in model years 1988 to 1995.  The PD Classic, (Black and Chrome - Scheme 640), was the last and was produced in 1995/6.
The PD had a unique 35 Litre Plastic Tank with a usual 1000cc airhead engine and 3.20:1 rear axle ration
For some countries, (not the UK), a R80GS PD was available with the only differences being the lack of an oil cooler on the R80 and a different rear axle ratio.
Apart from the Black and Chrome colour scheme the PD Classic had “round” rocker covers versus the rectangular one on the other PDs
A PD kit was available as an option to upgrade the normal R80/R100GS to the PD spec.
The model changed little over the years apart from the Tank decals, colour shemes, instruments, switch gear and rear shock.
The early models had early airhead switchgear on the bars, e.g. one switch indicators on the LHS whereas the later models had “K” switchgear, e.g. indicators both sides etc.
The early version, lets call it Mk 1, (my reference NOT a BMW one), had a white power rear shock as standard which was replaced on the two later versions by a OEM BMW shock.
If the Bikes are original the easiest way to tell the versions apart are the tank decal designs as these were different, the Mk 1 was inscribed “Paris Dakar”, whereas the Mk 2 and Mk 3 was inscribed "R100GS PD" as BMW removed the words “Paris Dakar” for copyright reasons, (reputed).  Apart from the lettering the Mk 1 and 2s had the same decal design whereas the last version had what is called a “pebble design”.
Other differences - Instruments, the Mk 1 had a big speedo and two other smaller dials, (rev counter and clock) plus a metal headlight grill and the later Bikes had a plastic headlight grill and two large dials, (speedo and rev counter).
The Bikes are great to ride and in standard form, can easily be used for long distance touring, light off road work, etc.  I have had a few and find them difficult to fault
Front brake, seat and screen changes are popular upgrades.
Dual plugging is also popular which "smooths out" the "roughness" of the 980cc engine

Quite a lot were produced and it would seem from the Frame numbers on Phil Hawksley's excellent site that the three versions MAY be differentiated by the number sequence;
Mk 1 = 641xxxx (Euro) and 613xxxx (US)
Mk 2 = 006xxxx (Euro) and 007xxxx (US)
Mk 2 = 034xxxx (Euro) and ???xxxx (US)
But as with other BM models you cannot take this a definite, (my views)

Two of my previous Bikes with different colour schemes............here..........and here and my 1989 Bike with the early tank and a few mods
E&OE

ClickForSlideShow