Yup, you're right. The H/L switch on PAL modulators adjusts the Audio subcarrier frequency.
PAL regions have different audio sub carrier frequency depending on which region.
H = 6MHz (UK, PAL-I)
L = 5.5MHz (Europe, PAL B/G)
Or maybe it's the other way around?
Anyhow it is the screw which adjusts the video UHF frequency on PAL modulators.
