My Meike kit also arrived yesterday. Everything works, however compared to the Fotodiox 15mm extension I got earlier, the Meike units definitively are much lighter and less sturdily built. Likely the material is some kind of plastic against the metal of the Fotodiox.
I did notice my 50/1.8Z can handle added extension much better than the 85/1.8Z, the latter develops visible amounts of spherical aberration, coma, loss of contrast and curvature of field. It's best to not go beyond the 11mm extension with this lens. This neatly illustrates a point I made made many times over the years: extension does impact mage quality and always in a negative manner, despite there is no glass inside. However, the carefully designed operational parameters of these advanced lenses are upset by the extension, and some lenses do respond very badly to this practice. Some lenses do better fortunately, but the outcome never is *improved* by added extension. Keep this frequently overlooked in mind -- always.