[canonuk]

The Bf-109E-4 and Bf-109E-7/N were the only ones that were tropicalised, thus the choice.

I'm sorry, but I have to disagree.
Correct me if I'm wrong, but the Bf 109E-7 "early" with DB 601A engine used 87 octane fuel. The Bf 109E-7/N "late" used 96 octane fuel.
I have two photos of Bf 109E-7 of 2/(H)14 and III/JG 27. Unfortunately my scanner gives no good resolution, but I can read on these 2 Emils in the little triangle "87" (meaning planes having a DB 601A engine with 87 octane fuel). Both are suited with Tropical radiator.
So this is not true, Bf 109E-7 with DB 601A engines were "tropicalized" too.
I will dig in and try to find close shots from Emils 7 with 87 octane fuel.

As for the reason why DB 601N behaved differently from DB601A, I don't know exactly. The author of the article I quoted above said "sand filters of DB 601N were quite unuasable". He didn't give exactly the reason why.

But the fact is that JG 27's DB601A engined (87 octane) Emils were usued until Bf 109F arrived late 1941, whereas DB601N engined (96 octane) JG26's Emils had a lot of technicals problems.
Perhpas their sand filters were different from JG27?

As I said, I will try to gather more informations.

Well, even if you happen to have a couple of photos of tropicalised 601A engines, they were definitely in the minority.

I'm the one who's built the sand filter for the Emils, and I've looked at dozens and dozens of photos of tropicalised Emils, and if the fuel triangle is visible it almost invariably says 'C3'.