That is not the stock HK buttpad. That is an E&L HK stock extension for the HK91/93. You can adjust it by adding additional spacers see the link below.There is that wheel/knob you can turn but it apparently does not do anything. I thought you loosened it to extend the stock.
The knob/wheel in your photo is part of the E&L HK Stock extension order# 500, not part of the original HK SG1 stock. There are additional spacers from E&L that will extend the E&L stock extension farther to give you additional eye relief. The description does not give the length but based on the photos it looks like if you used the full E&L extension the stock would be as long as the MSG90 stock or very close. I do not know what the knob is for because I do not have an E&L stock extension. I do have an SG1 stock and there is another one in the WTS section here, they do not have the knob shown in your photo.It is a standard G3/SG1 stock. You can add a cheek piece to it and control the height.
The only relevant item at the link is a Recoil Pad which I already have. It's barely enough due to the extensive eye relief of the scope.
Still not clear what the rotating wheel is for. You can rotate it clockwise and counter-clockwise with no apparent effect on anything.
E&L HK Stock extension #500: Adjustable Stock Extensions - Extends the stocks length to accommodate your personal requirements. Fits all HK91/G3 and HK-93/G33 firearms with plastic stocks.