I suspect (someone can maybe check the label) it is simply an oxalic acid base held in solution.
There are a number of manufacturers in the EU of various good products that work really well, but if you want to save money
you can easily make it yourself for pennies.
Oxalic acid crystals from a pool shop can be bought by the tub - they use it to remove stains from fibreglass pools. We've used diluted 1:100 with fresh water
we used it to remove water born rust stains from white hull
- painted and glass.
Mixed 1:200 it brightens teak
And mix it 1:10 with water and any stiffener to make a paste (we've used micro baloons OK) and you can dob it on stainless anywhere (turnbuckles - overhead stainless bots - wire - whatever) leave for 20 minutes before washing
off with fresh water. Great for getting into threads etc.
Remember it is an acid - so be sure to use gloves / glasses.