The Beeman HW97 is called HW97K in Europe, but it's the same gun (albeit on different power levels in some countries).
The Mk I was the version with the longer Barrel (37 cm) and shorter underlever. The diameter of the compression tube was 25 mm.
Somewhere in 1999, HW updated the internals of both the HW97(K) and the HW77/HW77K, and used a compression tube with a diameter of 26 mm which made the guns a bit more powerful. These were called the Mk II's and still had the longer barrel and shorter underlever.
In 2000 HW gave the HW97(K) a cosmetic update introducing a new stock and a shorter barrel (30 cm)with the flush underlever. Internally it remained the same as the MkII. This version is called the HW97(K) MkIII.
If you buy a new HW97(K) now, you will always get the MK III; some on-line shops still use the old picture of the MkI/MkII, but they will supply you with the Mk III.
To make thinks more confusing, in the UK (since you posted the same question on the airgunBBS), many are refering to the latest version of the HW97K as the MkII as there is a 12 fpe limit in the UK, so the MkI and MkII both had 12 fpe and wasn't regarded as an upgrade (although the larger compression tube was used as well).
I hope I remembered all details correct.