Our bodies should fit the Surefire KL4 head (and other Surefire E Series heads) fine, Duster1671.
Above is a photo that I just took right now with our E1 Classic body and a Surefire KL4 head unit, no lube added, threads on perfectly.
The Surefire KL4 head you have might have dirty threads, cleaning the threads and adding a bit of lube should solve the issue.
Feel free to contact me at the company email if you need anything for faster service as I don't log on to CPF as often as I check my emails.
About your questions regarding the threads on the 18650 bodies.
We beefed up the weak spot on the bodies where 18650 bored Surefire bodies happen to all break.
We have had no incidents of the bodies breaking as far as I know.
I sent a dozen bodies to CPFers to test for 2 months before release and asked them to use them as roughly (but not purposefully trying to break them obviously) and they all held up fine.
And yes, our future E Series bodies will be made in 18350 and 18650 configurations.
But I am not barring out making 16650 bodies again in the future if there is demand for them.