I bought a cased prewar fat- handled Tech in good condition a week ago. Out of curiosity, tried its handle with different heads. To my surprise, it worked with a DE89 head, which is metric. So, unlike my 1953 ball end Tech's handle which is 10-32, this old fat handle appeared to be M5 (the head is still 10-32).
10/32 and the metric thread size normally used for 3 piece razors will interchange in many razors.. I've actually only run into a few 3 piece razors that wouldn't interchange... Have you tried your BallEnd Tech handle with your EJ head or the EJ shave head with your Tech handle?
Oh, please use that "interchange" carefully when it comes to threaded male and female parts! For example 10-32 male will work with M5 female, but M5 male will not work with 10-32 female. So, I answered your question - ball end Tech's 10-32 handle will not work with DE89 M5 head. To be accurate that metric thread is called M5-0.8 (the numbers are for the thread outer diameter and the pitch in millimeters).
