We have a video that shows both the USM & USM-L above.
The main difference is the horizontal & angled weight that the USM and USM-L can tolerate before the Lyres sag.
The weights refer to the support the individual Lyre will give, in terms of sag when mounted horizontally - not the weight the mount can hold securely. When a heavy mic is used horizontally, or angled then the weight will make the inner ring sag towards the outer ring. This isn’t ideal, hence why we’ve recommend a max. horizontal weight. When used vertically for vocals, this issue is not a concern.
The soft-grip pads will hold well in excess of 2 kg.
Lyre Horizontal & Angled weight tolerance -
USM - 500g
USM-L - 250g
For more information, here is an article in SOS about the USM - http://www.soundonsound.com/sos/mar10/articles/rycoteinvisionusm.htm
Hope this helps!