Question reminded me of drawings I trashed yesterday while cleaning files. Some years back I used junker stocks with identifiable inspection/arsenal marks and measured and sketched the oval openings and distance between the openings at the bottom of the barrel channel. Some still had actions so series could be determined. Many makers are different and I would guess that as production progressed from series to series there were changes made by the same "maker."
The sketches had become "too cold" for me to do anything with them so I 'round filled' them. It would be a good project for someone with most series and variations within the series to disasemble the unstaked rifles and see what the innitial channel configuration was and where, what series, changes in the channel configurtion occured. Why, because it's an unanswered question.