Yup, that sounds like an 1/8" straight pipe thread which is a
variation of the more common tapered pipe thread. You should be able
to screw it to any standard 1/8" pipe fitting. So, I'd wager they were
using standard pipe as the rod and using a standard pipe thread for
the connections. Why use an odd thread pitch on pipe threads? Perhaps
to prevent people from attempting to screw standard fasteners into the
tapered threads. That would deter folks from trying to cap a pipe with
a standard screw, which would not seal well due to a lack of taper.

If memory serves me, garden hose threads, which are referred to as
3/4" GHT, are 1 1/16 by 11 1/2 TPI. Now there is an oddball thread...