Ok. This is what I get for relying on my memory -- and not sending the butt originally. The butt is 20.71 mm exactly at the point of connection with the shaft. The only stainless is on the butt. The shaft is 20.38 exactly at the point of connection with the butt. Because the shaft is smaller than the butt, the butt would have to be taken down to fit the shaft, which seems to me a really stupid idea. Only reasonable solution would be to send butt off to have another shaft made. Instead I'll try to make it myself. As I am newly retired and aspire to teach myself to build cues, the attempt to build the shaft would be good experience. What I've learned so far is that building a cue is anything but easy.
At any rate, I should have checked my memory before bothering you guys with my question. Apologies.