The message .com
from " contains
these words:

I have an old plumbing elbow, female both ends, it looks like half inch
bsp to me but is there a way to double check please. Just for info it
will thread on to a 15mm compression joint thread.
Thanks

PS The inside diameter is about three quarters inch

Half inch bsp is 14 turns per inch. My source confusingly quotes the
dimensions in metric:

Pitch 1.814
Major diameter 20.455
Minor diameter 18.631 (0.73")

--
Roger Chapman