So it is. I did find it, but I had to grep the source code to do it. Pity I didn't follow BR's link first off.
FYI for others:
It appears that the core feature version of '
Open with...' is only available:
a) By right-clicking the format link in Book Details--which also means it's invisible and inaccessible if you don't have 'Formats' checked in Preferences > Look & Feel > Book Details, or;
b) From the '
View Specific Format' (alt-v by default) action, under the
View action.