Tricky to tell from here without seeing / measuring, but I would have thought it was a BSP thread?

"It's the same diameter as a 1/4 BSP" - just one piece of advice (apologies if I'm teaching granny to suck eggs!) but 1/4"BSP, when measured, will measure closer to 1/2" - pipe fittings are different from standard screw threads in that regard, and it does confuse people.

It isn't those quick-fit fittings for flexible pipe by chance (used in some radiator circuits, and similar in apperance to plastic airlines)? Pictures would be useful if you can get some.