The solution to your question is very simple and was, in fact, included in my June 23 response to a somewhat similar question. Here is a scan of page 2 of Roy Wrigley's booklet on Canada's OHMS Officials.
Remember that this is a 'large' format stamp and would have been handled with the right hand side being uppermost (i.e. turn your illustration 90 degrees left) to perforate 10 impressions. Your left hand stamp then became thenext row to be perforated and it was struck twice.