As mentioned as part of the previously posted comments about these "auxiliary" postage meter markings, I think your first step would be to try and get hold of the August 7, 1978 issue of Canada Stamp News that apparently contains an article on the subject that would be interesting to acquire. (I believe the correct name of the publication is Canadian Stamp News:
http://canadianstampnews.com/the-ma...ne/magazine/As far as placing a value on covers, I don't think anyone can give you a firm answer, especially if you're using it for insurance purposes, as you can't just invent a number, but must have some basis for it. Maybe a check of
ebay sales on similar type metered mail from the 1930s may be a start.
Another thought that may be worth mentioning in your presentation is about the green ink used in the meter indicia shown. Although green ink has certainly been used in meters, it is not that common and that in itself could make the auxiliary marking in that color even more desirable.