I ordered another Neco headset from Aliexpress showing clearly the 2 silver washers in the product photo. 5 days later I received the headset and to my horror, the washers were missing again! I wrote to the supplier with my complaint and they explained to me that only the old headsets came with the washers, the new headsets do not need the washers. I didn't believe them and argued with them. But to my amazement, after fitting the new headset to my IP-256SL, it was a perfect fit! The top cover no longer rubs against the frame. I ordered the headset from Vivian here:

http://www.aliexpress.com/item/Neco-Expander-Cap-Neco-Headset-1-1-8-to-1-1-2-1-5-Integrated-for/1698419284.html