On taking a close look at the latest 'enlarged scans you posted... it appears that there has been some 'double exposures' made. Observe the gentleman sitting at the board on frame 15... Below his hand... at chair level there seems to be an image of a partial right foot. Since I have little medical anatomy experience... I have to question from "where could this 'foot' have come. other than by double exposure. To my eye, the 'regular' surge type marks around the sprocket holes are another other 'mystery' that I will leave open to the conjecture of someone else with a 'better eye'. My guess is that this film has gone through the camera more than once... but the first time, perhaps somewhat underexposed, but not 'fully re-wound back into the casette. The ''regular pattern' of what we (or I) can observe... perhaps as a 'closed' shower curtain might (when 'added' to the evidence of 'the foot') indicate a previous 'trip' through your camera.