Help with adding unique codes to numbers

Hi all.

Could someone please help with a query.
Is it possible to automatically add unique codes against specific numbers.
As you can see from the document in column A, we have a lot of stock delivered and each pallet has a unique 7 digit number.
Each item from that pallet has to have a label attached.
Rather than adding the number, I have been giving each pallet a unique 2 digit letter in column B which makes it quicker to process the stock.
Is it possible to generate the letters automatically so I don't have to keep filtering each pallet number and then giving it a two digit letter.
As the first pallet I have given the letters PA, I just want to copy down and all subsequent numbers will be given the same reference.
Then the next set of numbers will be given PB and so on.

Hope I am making some sort of sense.

Thanks for looking.

Graham

That could be 3 or 4 weeks until we get to Z
I would start back at AA again.

Graham

We have a sale sheet where each item is given a lot number. When the sale is being typed in, it is much quicker to enter two letters than a 7 digit number.
It cuts down on errors when typing in. When the sale is completed, I convert the codes back to numbers to send off to the Vendor so they can match the items to their manifest.
Hope this makes sense.

Graham

The sale sheet has multiple columns, each with a formula referencing something else within the workbook and it would take longer to click a dropdown and select the code than it would just to enter the letters.

Graham

Try this in B2:

=LEFT(ADDRESS(1,27+MOD(389+SUMPRODUCT(1/COUNTIF($A$2:A2,$A$2:A2)),676),4),2)

Thanks Phuocam.
This is what I wanted.

Graham