Kenfuscious:  I checked the threads on the Pacific K-2 'upper' with my thread gauge.  The thread accepted the 24tpi and 26tpi blades closely, but not fully.  I'm guessing the elevation shaft may be 25tpi.  Unusual, but may make sense in a micrometer adjustment.  The maximum diameter of the threads is .185".  I tried a 10-24 screw in the threaded hole in the base.  It engaged several threads before jamming.  I believe the K-2 sight used a peculiar thread.  I have also noticed that there are surprising variations in K-1 Pacific bases which effects hole positioning.  There must have been little gauging and a lot of hand fitting when they made these sights.  I would recommend getting a Pacific sight as a complete unit or at least trying an 'upper' on your base for function, before buying.