John,
 And just to add one further note to those of Tracy Smith and John Brown, it was only during the period 6 Apr 98 - 7 Apr 99 that the significant level data was not used. As John says, the RUC1 used sig-level data throughout its history from 94-98.
 The missed sig-level data for this 366-day period is only for the NCEP RUC. If you are using grids from the FSL MAPS runs archived at NCEP, this was not a problem. The glitch occurred due to a slight difference in ordering of mandatory and significant level data at FSL vs. NCEP, which caused the NCEP RUC analysis program to not properly merge in the sig-level data for that period.
 - Stan