Try a MK 5, really simple or a mk2. I just did a MK5 locked right up as has every other one I’ve done.
Not so interesting I'm afraid. I've owned two new MK25s and a new Atomic Z2. All three gave me trouble right out of the box. I tried to service a MK10 Plus but had so many creeping issues I eventually gave up on it. In comparison I've owned numerous diaphragm regs and have serviced well over 30 I guess, probably more, and not one of them has ever given me any trouble. A small sample of piston regs I'll admit but enough to put me off them.