If that is the case then according to the 1988 Standard the root dia. should be between 0.8600" and 0.8406" and it will be 5 tpi....it says it has a 1” acme. I think however your suggesting of a stub acme sounds spot on.
Whether or not you are able to measure the root dia. on the spindle in question is debatable but the 5 tpi could be relatively easy.