The first part is you need to make other items with the same item No. as Default:= false for that you can use here the ModifyAll function Like the bellow
write this code to Field "Default" OnValidate Trigger in Table
var
RecItemModel: Record "Item Model";
RecItemModel .Reset();
RecItemModel .Setrange("Item No.", Rec."Item No.");
RecItemModel .Setfilter("Make Code",'<>%1', Rec."Make Code"); // if this is unique for other entries with the same item no otherwise this will not work try to use Primary Keys that makes the entries unique to prevent the current record to be modified
if RecItemModel .findset then
RecItemModel.Modifyall("Default",false);
the editable or noneditable thing can be done with the help of Boolean variables on the Page level share your objects code if you still face issues to achieve this I will try to help you out in that as well.
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