Software

Look up append queries and delete if you mean take the record out of tbl1 and puts it in tblStockOut.

You aren't thinking sets here.
In a full DBMS you could do this with
SQL and a while loop, but you would try to avoid it.

For instance what happens if you run the thing again. Dulplicates , invalid primary key...
Dealing with that means checking to see if you already have, or complicated logic to check whether you should.
Then there's knock on populate tbl1 move to tblStockOut, now if you only check tbl1 the populate will succeed, but move will fail later. So do you overrwite, skip or fail at that point, check both (and any others that come along)

Tin of worms that...

So why do you need to prepopulate and why do you need to move?

If you had a table productRanges, your show product code could show it if it existed or create it if it didn't but the range was valid then show it.

Equally the move operation could be just setting another column to indicate the operation had occurred and the row was in a different state.
Thats just
Update Stock set Moved = true where sequence between 301 and 400
301 and 400 coming from fields on your form.

Aside from performance reasons splitting teh same sort of data between two or more tables is a bad move.

What if you want a query from both now you are in a situation where you need a union query...