Nathan
Before V4.5.5 the 7/6 correction was done after the common Pb correction. So, if you had the 207/206 correction set to use the main reference material it effectively did nothing since the main reference material had already been corrected to have the 7/6 specified in the RM data. For version >= 4.5.5 the 7/6 correction is done before the common Pb correction. So, if you use the primary RM (that has variable common Pb, otherwise why use UcomPbine?) you will get a very strange 7/6 correction. If you were fine using the RM for 7/6 before, then you should now use none. Does that change explain what you were seeing?
All the best,
