What is "rf" on a parweld regulator

[bigegg]

This is probably a daft question, but a quick Google search turns up nothing apparent...

I have just bought a parweld argon regulator.
It had LP HP in out and RF coupler.
The others are obvious, but what is RF?
It's blanked off with a screw in cap.
I thought at first "flowmeter" but apparently that would fit on the "out" connector, before the hose.

Ta

 

[WorkshopChris]

Not sure about the 'RF' in this situation but occasionally regulators have a blanked off port that is a second HP port as they use the same body to produce a side or top entry regulator. BOC cylinders are top entry and many of the rent free option use side entry.
Occasionally a port is blanked off and serves no purpose other than allowing a entry point for cross drilling during manufacturing.