Its something thats left over from days gone by when the reference was mainly for cylinder heads. stage 1 port and polish, stage 2 large inlet valves std exhaust, stage 3 Large inlet/exhaust valve ultimate spec for the day. HTH
If you use reprofiled cams you can fit the K12 valves which are the same size but are 2mm longer enabling you to fit the standard shims.
I know someone who has changed the 773's because they didnt have much go . Will know what his second choice will be like after the weekends dyno session.