I am getting ready to build my SatNOGs and I was mapping out the holes and cuts on the C1001 parts. The diagram (File:C1001.png - SatNOGS Wiki) shows holes being drilled (4 in total, 90 degrees) at 96 mm (-1 for the cross beam) and 40 mm (-3 for the base/tower connection). The instructions on the document also call out for C1021-1 to be moved from the Zero Ref point to 7.5 mm and holes drilled. This seems incorrect as that would put it in L cut for mounting the extension for the antenna. Going back from L 7.5 mm also does not make a lot of sense.
I think I figured it out. I was expecting another set of holes for the homing/encoding ring. It does not look like those are called out in the document (either due to it relating to the old homing method of a clamp type or to allow for adjustability). The 7.5 mm is due to the collar that is used to help line up the holes (align at Zero-Ref, go the length of the variance drilling, and then come back 7.5 mm to line up the holes where they need to be). As I mention, the lack of second set of holes is what threw me off.