A quick look and they are bother part of the same company.
The catalog number relates to how they are selected from stock. The descriptions are the same so I would say they are the same item. Its like buying from PC World and Currys same company but sometimes different prices.
Additions is part of the Great Universal Stores group, who belong to the Barclay Brothers if memory serves.
The price difference is down to how the companies operated historically.
Great Universal Stores was, and probably still is, a mail order catalogue club where you used to pay for stuff weekly, the catalogue price included the interest for 20 weeks payments or whatever.
Additions was originally spun off from Argos, so the prices were the standard store price, and interest was added if you paid weekly.
We had the same problem when I worked at Index, our Index Extra catalogue was the same as the many Littlewoods Home Shopping catalogues, but the catalogue prices were much lower, and I often had to explain to customers why this was...