As mentioned in an earlier post, October 24th is significant to the United Nations as it is "United Nations Day" each year:
United Nations Day isn't solely an American holiday but a holiday celebrating all of the peoples that came together to form the United Nations. It was the United Nations General Assembly that established the holiday in 1948. The date set for the holiday is October 24th, the same day that the United Nations Charter was adopted in 1945.
There are also various learning institutions and international schools across the world that recognize and celebrate United Nations Day as a day to show pride in their cultural diversity.
Based on the above quote, I suspect a reasonable lead would suggest that a Fond du Lac school or college/university may have been the user of that postage meter. (I wonder if there was any Fond du Lac citizenry connected with the United Nations that would have connected the dots, so to speak?)
In any event, I very much doubt the USPS or Pitney Bowes would have Postage Meter Numbers, dating back more than a half century. I suspect that once a meter is "retired" and replaced with a newer model, all records on the old meter are destroyed along with the obsolete equipment.
Although it may be possible that some historical records do exist on these old postage meter users, in this day and age, I would imagine it would be a question of both privacy and protection to their business clientele that would result in the USPS and/or Pitney Bowes choosing not disclose any of this information to the general public, even if the data was readily available.
As to the final question about the $0.00 value meter imprint from October 24, 1960, I would think this would be a test only and not legitimately recognized by the philatelic community as a commemorative cover. Even today, any mail clerk could print hundreds or even thousands of any $0.00 valued meter imprints if they want to waste the time and money to do it. The fact that the covers are of zero monetary value for postage and therefore did not see any postal use, would render them worthless, or merely a curiosity at best, to most collectors. The meters that did register a value and legitimately went through the mailstream would, of course, be more desirable.