Serial number range- starting with Alpha or Number

Hi friends

We are generating serial numbers against the production orders. We have maintained one internal number range (only numbers) and one external number range (Alpha numeric-.A000* to Z999*)

I am able to generate serial numbers starting with alphabet (eg-A1234) but unable to generate serial numbers starting with numbers (eg 1AB001)

Can you please guide me if this can be achieved

Thanks & Regards

Pradeep