I think it is an electro-mechanical actuatorI don't know where exactly to put this, but since it's tangentially related to DF-17, I'll put it here.
Images from the show booth of Wuxi Elite Linear Motion Machinery Corporation at Zhuhai Airshow. The company is apparently the provider of hydraulic telescoping cylinders for missile TEL vehicles, including that of the DF-17 TEL, which you can see on display at the booth.
