The design of the one I was asking about seems to be called "stacked letters", where the first two initials are placed on over the other, and the last initial is to the right of them. Overall, the monogram is called a "circle" monogram, where the initials are squeezed to fit in a circle. I didn't find any that looked like stacked-letters in a circle to show what kind of individual letters we might find in others of the type.

I'm curious where you found Montclair as a typeface style. I could only Google up references to the style manual of a Montclair University in New Jersey (which only presented well known type faces and defined how they were to be used in University pubs).