Steve ,
That noble metal chart shows that the aluminium alloys range covers performance that may only be slightly better than mild steel up to nearly as non noble as Zinc.
Basically you need a specific Anode alloy of Aluminium to be effective hence you need to be careful where and what you purchase.
I assume unbranded cheapies won't meet the required specs and really expensive ones should hopefully exceed the specs however ,
there is a big difference in prices between suppliers/brands e.g Mcduff £23 solent anodes £7 for seemingly the same style
I was able to find a datasheet from another supplier which gives a breakdown of constituent parts it might be useful to check against alternative suppliers before deciding on where and what to purchase
datasheet
Make sure whatever you purchase conforms to the correct MIL specs for sacrificial anodes and if they can't supply the spec ratings or datasheets walk away .....teaching you to suck eggs I know but better to be safe than sorry.... There's a lot of cr*p on the market
Peteg