From "Locomotive Dictionary (1906)" reprint 1972 :
"Inside Admission Valve: A valve which takes live steam through a central cavity and admits it to the steam ports leading to the ends of the cylinder. The exaust ports are at the end of the valve."
"Outside admission valve: A valve which admits steam to the cylinder through ports which are uncovered by the outside edges of its ends of the valve and opens the exaust ports by the inside edges of its ends."
Rather Chinese for me but I hope it helps.
Hans R. Banziger