I stripped down an EMax BL2820/07 Electric motor, so that I could remove the shaft - to part off surplus length on my lathe. When reassembling I discovered a part I had not noticed on strip down

It is a small piece of phosphor bronze tube - OD 6mm, ID 5mm, Length 1mm. It slips neatly over the drive shaft. I can't see any purpose for it - and if I fit it internally, it creates what seems an excessive gap between revolving and fixed parts of the motor. I guess it might fit between the Eclip and the back of the motor - but it seems unnecessary there.
Could anybody tell me where it should fit?