Well to be clear only the female on it is brass. I have seen a few over the years but it is the first I have owned. I am curious was this factory? Looks like it was based on quality of the cast. Was there a specific company did this or reason why? I have started cleaning it (I didn’t put the chop or grinder marks on it) but I know many post vises didn’t have maker’s marks.  

Jaws are 5” haven’t weighted but I guess 80lbs 

I think it's a repair. Someone had the vise, worn the treads down (altough that usually happens on the male side of the screw), or most likely simply lost that part.  After that it's just a matter of making a mold, casting a cylindrical object in brass, then use a lathe to do the rest.  I've seen one be made from aluminium entirely on a lathe. 

Nice vise tough. 

I have an older vise with a bronze or brass screwbox on it . I’ve seen 2or 3 more of these