Trumpy, I think the PoCos like multiple transformers (of essentially the same type) that they can mix and match in several ways to create all of the strange service arrangements we have. The service truck does not need to carry many different types to fix almost anything.
The 50 KVA unit is becoming the defacto standard around here.
There are several ways to tap them and it has a voltage select switch to enhance that adaptability.
http://gfretwell.com/electrical/Transformer%20connection%20label.jpg
