Since it says 7/8-18 NF that means it's "national fine" supposedly a standard thread size. Go to an automotive store to buy a bolt/nut in that size and try on your receiver/barrel. If its from Snap-On personally find the price unrealistic so its probably either inported or a chaser/restorer. Did some checking with my sources and that is a special thread tap size. Here's the URL for the source:

http://www.toolsavings.com/default.a...rchWithin.y=10

As you'll note they are imports and were either from Poland, Chezovokia, Pakistan, India or Chine the last ones I bought. Most have given me good service for my needs, i.e. few time usages, but suspect they would not hold up in production settings. This outfit used to be called Field Tool Supply and information was given to me years ago by Brownells tech people.