You're dealing with 2 different keyways. IIRC, Corbin Russwin has the most keyways of any American lock manufacturer. The L4 is their new stock keyway (comes in all new locks unless otherwise specified). The RU4 is a generic blank for the Russwin 981, an older stock keyway.
The depth and spacing specs are different between the two classes of keyways...
Look here for more information:
http://www.assaabloydss.com/library/par ... Manual.pdf