Darrell,
I don’t think that there is an “I” prefix on your tank plate. I believe that it is the number 1. For the 1972 DS plates on eBay (now sold), the paper packaging shows the plate number as DS 1869 whereas the plate reads DS 1-869. The Wikipedia page seems to indicate that for most period series of NB plates, the number would start at 1-000, and then increase:
https://en.wikipedia.org/wiki/Vehicl..._New_Brunswick
I have found another excellent video on YouTube. It was produced by the RCH in 1963. It has plenty of in-focus footage of Shermans in Petawawa, and Farnham. The footage seems to confirm that Ontario and Quebec did not require provincial plates on Tanks, but they were present on wheeled vehicles. The film also includes Worthy dancing at a Ball!
https://www.youtube.com/watch?v=Q1dya3vbszo