without that info we can't even begin to speculate on what the remaining mortises were for (if anything).

Oh yes we can!
(Well it is Panto season)

Or at least, I can speculate they weren't cock ups.
To build the thing you'd have to build the ring first and lay it on the ground ready assembled, upside down, and only add the mortices when you'd added the uprights and were sure of the exact position of the tenons.

It seems very unlikely you'd then get the position of the holes very wrong (bearing in mind you's have the option to rock the uprights quite a bit to do the final fit, MFI assembly style, and in fact the design almost calls for that as its strength would be all the greater that way).