Made the shower like new.
The replacement cart was fitted in about 5-10 minutes, and the shower works better than ever, with a lovely smooth action on the water control, and fast reaction to temperature changes.
The instructions with the cart were clear, and it was nice it came with spare seals (although not needed).
The service from Shower Doctor was very good as well, both in the speed at which the order was sent (I ordered late Monday, and had the cart Wednesday), and the FAQ section helpful.
P.S.
I saw someone asking about a part 410.12 - I think that's the part number for the red high/low pressure adjustment plate on the bottom of the 903.33, as I saw a similar number on our old cart and it threw me a bit.