The Unger no 1 will fit the end of the harris, and the no 4 fits over the bottom if you take the black plastic off.
i just took a old worn out pole, stripped it down to bits, cut it in 3 section 3ft long got some new clamp levers glued it all back together, its great for shops, bungalows, i wouldn't be without it.
I bought two Harris poles today for £30 for both. I have a 'sheeth' that my brush head fits on (Emporium used to sell them but I don't think you can buy them now) and all I have to do is hacksaw the top bit of the Harris and my brush head just slides on the pole.Takes under thirty seconds to do; depending on the hacksaw blade.It's a nice pole for banging about on domestic work.