Low wood was early production, before someone changed the drawings to reflect the fact that the no4 production rifles did not have a mag cut off. The trials rifles from the 30s did, but this was dropped by the 40s. The wood may have been original to the 20L rifle shown, or it may have been recycled from other rifles under repair.
I picked up a bunch of low cut wood a number of years back, and observed serial numbers up to around 17L or higher with the low cut. Of course, with LB production changes, trying to pinpoint a definite cut-off point to anything usually has variables.