Find Highest Letter (a being the lowest) from table

Hi, I have a product code ABC followed by a single letter a,b,c. A product can be:

ABCa

ABCb

ABCc

If there is a 'b; ending there must be an 'a' as well etc.

I strip out the product SKU using:

let a := length(Image);
let b := 70;
let d := substr(Image, b, a - 5 - b);
d

The letter at the end is in another column using code:

let a := length(Image);
let b := substr(Image, a - 5, 1);
b

I want to know per SKU what the highest letter it goes to (a,b or c).

Any ideas?

Thanks

Matt