To group 2 first series number

Hi,

I would like to group my sales delivery details based on 1st free numbers.
For example i have series for my sales order;
A01 - SO Alz
A02 - SR Alz
A03 - SO Azt
A04 - SR Azt

therefore, in details i have done;
model = tcms050.desc (3,10)
so it able to print "Alz" for series that start with A01 and A02 and "Azt" for series A03 and A04 only.

And then i wanted to group it by model.
I tried to declared "model" as input field and create after.field.model at report layout. but it is not working.

may i know how?
your advice is much appreciated.
Thank you.