a very interesting joint today. Chinese/Japanese in origin, it is
>used to join wood end to end,
In English timber-framing practice, this is an edge-halved scarf joint.
It's typically regarded as mid-14th to mid-16th century work. Before this a splayed scarf joint was used, afterwards a face-halved scarf joint. All three are widely seen and are a valuable means of dating old timber-framed buildings.
For the splayed scarf, (the mating faces are sloped, and in two parallel planes. It's locked by a rectangular peg (or pair of wedges) knocked into the gap between the two steps in the sloped faces. They're hard to lay out. and take a while to cut because it's a sloped-grain surface. After the Black Death had killed 1/4 of the population, there was need for a simpler joint that the surviving carpenters could have cut by unskilled labourers.
The face-halved scarf is even simpler to cut. Imagine turning the large flat surface by 1/4 turn, relative to those stopped mortices. Now the whole thing can be marked on a single face and sawn out without chiselling into a stopped corner. The face-halved joint is much less strong (it's still strong, but only in one direction) but it can be made by even less skilled carpenter's labourers.
A couple of weeks ago I was at Cressing Temple barns in Essex. These are a pair of 13th century timber framed barns and show both styles of halved scarf joint. Pretty long timbers too - some are over 40' in one piece.