Old cashback offers - can anyone remember when they applied?

I think the earliest cashback was 2%, falling to 1% after a while. When I joined the loan numbers were in the early 200s and the cashback was 0.5% routinely with occasional promotional rates of 1% or 1.5% (usually in late June IIRC).

If you routinely bid the same amount then does it really matter which cashback is which - simply apply all cashback in loan number order until you run out. Cashback promotions started and ended on a specific loan number, so the first cashback that appears on your statement will apply to the highest loan number you held at that time, and although new dollops of cashback will appear in the order loans were drawn down (not necessarily in loan-number order), you will have received cashback on every loan part in number order up until you run out of cashback dollops. If you didn't routinely bid the same amount on each loan, you should be able to work out the loan part from size of your cashback from the size of your bid.

Alternatively, don't view the cashback as associated with a specific loan, see it more as a self-managed provision fund that you can use to cover some of your earliest losses.