Yup - tht's the one. Unlike the baby carrier, I've seen it on sale, it really exists. The accessory catelogue says that it's suitable for children weighing from 9kg and up - although omits to mention the upper weight limit, I'd guess at around 24kg (50lbs approx). Once they are about 18kg (approx 35lbs) the restraint tray is no longer required. Weights are more important than ages when choosing car seats.
Although the items shown in the catelogue DON'T quite match the official price list
The catelogue describes Junior Seat I+II as one item - the price list gives one entry for a "Junior seat I+II" and a separate entry for "Junior seat II". Junior seat III is a booster cushion, BTW.
Part numbers differ between ISOFIX versions and non-ISOFIX versions. You pay a LOT more for the ISOFIX variety, so BMW is happy to charge a premium for safety

Ordered my "S" August '01. Arrived July '02. EB with white roof, wheels and stripes, Chili pack, DSC and climate control. Hate those 17" runflats though.
Can now be seen all over the West Midlands, as a blue blur passing everything at speed