Are you sure? From what I can remember the round one came out first, and was replaced by the rectangular version around 1986.

The round one looks like a small version of the 28w one. Can it be opened up to see the ballast like the 28w?

The round one has 2DA16 Mk2 on it, the square one just has 2DA16?

I checked the old Thorn catalogues and believe the first version was the plain round one shown in the 1983 catalogue, page 113. Then your round one in the 1984 catalogue, p.116. Finally the rectangular one in the 1987 catalogue, p.91. Perhaps the Mk2 on the sticker just means Mk.2 of that particular model.

I stand corrected lol. Cheers for the info