The "53" could just as easily have been a typo in the advertising material, but not regarded as significant enough to bin the run and reprint them.
Also, although the "53" does not seem to make sense for the known researched dates, perhaps it is possible that Paul Jokelson funded advertising material later than 1975.
And, for more interest, note that in the last period, the company was "Highland Paperweights Ltd", but the labels stated "Highland Paper Weight" ... with "paper" and "weight" as separate words. Does that mean anything? Maybe not!