It is definately the 1924 pattern - as is the similar KD frock on this post:http://www.victorianwars.com/viewtopic.php?f=19&t=7092
The simplest way of determining this is by the central vertical seam running along the back - earlier versions have the double 'fishtail' seams. There is a trader on Ebay who regularly sells these as 'WWI pattern' - I suspect he knows better.
I had another post on this forum (under AndrewM - I've been away for some time and lost my password, hence the new username), where I posted some photos:
viewtopic.php?f=19&t=1196Andrew