The Charlton Standard Catalogue of Caithness Paperweights lists items as "Style One" & "Style Two" etc. where a later variation in the basic design was produced.
For Sentinel, the Style Two version was a commission by Royal Mint Exclusives. The "Style One" was an original, unlimited version produced in 1975 (or 1974 if referring to the 1st edition of the Catalogue book). The original was design number CT40 (they stopped using design numbers in 1998) and was made in Green, Purple and Red.
Della's example is the early one and had an original issue price of Â£12. The 2nd edition of the Catalogue book gives a 2004 UK "value" of Â£40.
I note that Della's has less height in the central "core" below the bubble than is hsown in the books. However, it is likely that with that unlimited issue, there will have been many variations in details such as height and other shapes within the internal design.