I have a delightful table together by Andy Pugh of Bodgesoc Industries,
Sheffield, UK showing 526 standard thread sizes up to 6" . Closest it
comes to yours is a 16-18 ASME Thread with .268" OD and 18 threads

Don Young wrote:
Going through some old taps I found a tap marked 1/4"-18. I know that is the
designation for a standard NPT tap but this tap is actually 1/4" in
diameter. Looks like a 1/4"-20 with coarser threads. Does anyone know of an
application for such a thread? I suspect it might have been made mistakenly
from a list of sizes which included the pipe thread. No manufacturer
identification and appears to be a cheap import quality.

Don Young