I have one and it works quite well. One limitation is on the male thread - you can't get a thread right down to a shoulder - if you see what I mean - there's quite a gap, as the shouldered part prevents all the tenon getting into the die cutting part.
Apologies if Axminster has altered their design since I bought my set of thread boxes about 15 years ago, but there is an entry guide consisting of a wooden block on the face of the box that guides the dowel into the cutter box.
It’s held in place on the entry face of the screw box by a couple of screws and when removed, gives access to the vee cutter underneath.
In use, the cutter cuts the thread first and it is held in place and advanced at the correct rate by a metal thread behind that follows on the thread you have just cut.
I have successfully cut threads to within a gnat’s nibbler of a shoulder by starting the thread in the normal way until reaching the shoulder, where it stops. You then unscrew it, and remove this block to complete the last 3/4” or so.
In this way the cutter is able to travel right up to the shoulder. You will need to tidy up the ragged end of the thread with a small pointed knife.
For a nice finish, return it to the lathe if possible, and undercut a groove at the end of the thread beside the shoulder.
Whatever you do DON’T alter the Vee cutter’s position. They are a first class pig to get right if moved.
Hope this helps