Finn M39 "Finger" splice anatomy

[WarPig1976]

How are the uniform tapers accomplished?

Picture a large disk with finger like cutters. You feed the piece in from a jig that holds everything square until you reach your depth of cut.
Router probably isn't the right description. A dedicated milling machine is more like it. It's a very complex operation if you consider the two joined pieces have to come out to an exact measurement.
Without a large expensive custom made cutter. A dedicated machine to spin it and jigs that have been well thought out it's just impractical to try and replicate today.


Here's a video of a finger joint cutter on a small scale
.