G'day Nigel,

Looking at the first of Mitch's links confirms my first reaction: your "... 20¼" wide by 14" high ... measured at the open centre page, i.e. a double page)..." is pretty much a trimmed half Imperial, i.e. music that was printed 2-up (four sheets to a side) onto Imperial's 29½"× 21½", folded twice (to 21½" x 14¾") then trimmed ¾" on each of three sides.

Unless you come across uncut examples of music - printed as weekly (or some other interval) part-works - to be bound by the subscriber's local bookbinder ... you won't find sheet music in full correspondence with the original sheet sizes.

Regards,

Bob