"....303-6mm..." Likely 6 mm Musgrave. Used for springbok in South Africa. A necked down .303 British. Launches an 85 grain bullet at around 3000 fps. No idea where you'd get proper load data though.
You'll have to do a chamber cast to be sure. The rifle hardly looks big enough for a 2.222" cartridge. Custom rifle. Custom ammo. Custom dies.
There's a set of Lee dies here.
http://www.titanreloading.com/6mm-mu...-die-set-steel
RCBS makes 'em too, but only on Special order and horrendously expensive.