There is a Pervasive utility that can merge 2 files of the same data structure together which would accomplish what you need to do.  Copy your 12/19 backup file into the same folder as your active data with a different extension so you have (for example) both INVTXN.B and INVTXN.BAK in the folder.  Then in the Pervasive utilities, choose Maintenance - Data - Copy and copy from INVTXN.BAK to INVTXN.B.

That's a good question.  I know they are a new IS Tech Support subscriber and had to run DBA update from a prior version before the IS Tech updates could be installed.  The DBA Update did rebuild the INVTXN file back several versions ago so if they were updating from something really old, it may have had a problem with the rebuild.  Recent DBA updates and the IS Tech updates have not rebuilt the INVTXN file.