Hi Mike,
I'll ask Rob who is a FFY specialist. My guess is that they should have been dressed almost the same manner as 19th (L&BH) Armoured Car Coy, RTC, which is WWI tunic & belt, collar dogs, black beret and cap badge. I'll ask him and will post here as soon as I know.

Moreover, if you're interested in the FFY during WWI and want to see some very pictures, maps etc, here is the direct link to read and see The Fife and Forfar Yeomanry and 14th (F. & F. Yeo.) Battn. R.H. 1914-1919 by D. D. Ogilvie

http://www.gutenberg.org/files/18468...-h/18468-h.htm

All the best
Phil