Any idea why Boeing made it work at Flap10?
Great question, sorry I didn’t pick up on it earlier. I have just had a look in the 737CL AMM and the electrical schematic for PTU (29-22-00 Page 8 for those with access). It shows that one of the switches to activate the PTU is the LAND WRN SW which closes when the t/e flaps are less than 15. NB it is the same in the 737NG AMM (29-25-00 Page 17). So it appears to me that Boeing have used the same switch that is used for the landing config warning horn. Since the criteria are more restrictive, ie <15 and not just up to and including 5 this is safe. I would guess this was for reducing cost & complexity, if not redundancy!