On Monday, October 5, 2020 at 7:36:41 AM UTC-4, Jim Wilkins wrote:

The search term is "steel rule".
http://www.wagnerdiesupply.com/Image...Guide-2016.pdf

PT stands for Point, 1/72", the traditional measuring unit of the printing
trade.

The cartons were labeled 'Die Knife' back then. Either the name was changed, or their suppliers used a trade name for that industry which isn't unusual. I haven't been in a corrugated box plant in over 30 years. There were five where I lived, in Ohio, but there are none close to me, here in Florida. The ones on Ebay were in a carton marked Die Knife. That was the search term that I used. It was a partial carton, and no telling how old it was. There is little to no reason to date code the shipping cartons.