Square D unloader valve thread?

I am building a new compressor from a salvaged C-H tank and other parts, the

99.99 5 hp pump from HF, and a new 5 hp, 1 ph motor. The C-H pressure switch is a Square D, with an unloader. The 1/4" tubing nut for the unloader valve is missing. I assumed that this would be a standard 1/4" tubing nut. Wrong. I then thought it might be a 1/8" NPT thread, perhaps a straight version, because the size seemed about right. Wrong again. Anyone know how to get a nut to attach 1/4: tubing to this unloader valve? I'd rather not have to buy one from Sears.com, or to machine one from stock. Is this some standard tubing fitting that I'm unaware of? Thanks.