What kind of union fitting is this? Ford Fitting

Please see attached pics.

This union fitting was given to me by the Seattle Public Utilities to make a connection between my new PE run (with male brass end) to the water meter pig tail. The pig tail is supposed to be 3/4" copper... it's pretty gunked up and the plan was to cut off the old connection and put the new union they would give me, which I expected to be a compression fitting. It may well be that but I am confused by the threading inside. What is that for?

So can someone tell me exactly how this thing is supposed to work?

Thanks.

http://www.fordmeterbox.com/pages/H_Section/Hsec.htm

The end with the bolt goes on the incoming pipe. There is a rubber gasket in there that makes the seal and you tighten the bolt down. The ribbing helps to grip the pipe. The other end is female threaded so's you can adapt any type of supply piping to it.

Thanks Tom... only got a quick look at it today and thought it was threaded... ribbed makes a lot more sense.