What it is saying I believe is that by 84 in the USA and 85-UK the 1.8 GTI engine became the standard power plant for the "GTI" Cabby models, which succeeded the Gli.
It's not saying that models prior could not be a GTI, there were a lot of quick changes engine wise over a few short years, you had the GLi etc beforehand which still had an injected engine albeit a 1.6. A 1.8 GTI engine was available as early as 82 for the GLi. So you could get both models with a 1.8 GTI engine.
Ah see I knew the GTI lump was used in the GLI's but I thought you could get a GTI spec cabby from 83... 
You could, but also a GLi etc, it was my understanding, only till 85 that they made the cabby GTI 1.8 definitive, I.E. all Cabby GTI would be the 1.8, have a look on some other cabbies, they have the early style badges as fitted onto the tin top. I believe the others were dropped bar from the GL, which then had a 1.6 engine fitted. Some of the changes made were across the hard top platform such as engine changes, dash etc, Grilles with twin spots etc.
The problem is VW made different levels over a small number of years, So putting it into context 79-83 is only four years for a lot of changes. So by 6 years the Cabby GTI was just that. To add to the confusion different areas had different designation, I.E. the American Market got the Rabbit, named because it was the Chinese new year animal at the time. It was confusing enough as the the cars were named after winds, the jetta (jet stream) the sirocco (North African, Mediterranean wind) and the Gulf (gulf stream) in German became the Golf in English etc, nothing to do with the game. Although it was quickly adapted with quirks like the Golf ball Knobs on later cars. Even the later Bora is a wind Google it you'll see.
Also cars from other countries have ended up here with different models over the years so some have confused as to what was available in the UK Market.
Things like the Rivage are called the Classic line in Europe and classic edition is another name for it used in The USA. I have seen some on E bay. UK, inc a Ettienne Aiger who is a fashion designer etc. The UK market didn't really get that much in the way of special editions as in Europe and the states.
I think that some of the refinements came across at a similar time they introduced the last Special Editioned tin top which was only ever called a special edition, but came to be known as the "campaign Model" becuse it was part of a VW campaign, others call it the "Pirelli edtion" because of the P Slots fitted.
(courtesy of dubboy).
It's also not uncommon for people to change colours, interiors and badges to update their cars. So this can add to the confusion also.
Read quite a few E Bay ads and you'll get what I mean "Special Karmann edition" ring a bell. Again no such thing as all the cars were built by Karmann. All the cars built there had the Karmann badges fitted as a kind of mark of respect. From research only the power train and suspension set ups were provided by VW.
The info is out there... You just gotta look.
