They said it had something to do with the serial number?
Although Russian SKS use an alpha-numeric numbering system, not all letters transliterate and are dropped from the registration as a result. When there are serial numbers of 5 digits or less and no letters, the firearms will automatically go to secondary review to check for number duplications.
Likewise, if a multiple firearm transfer is initiated and one or more firearm falls into the serial number criteria, all the firearms on that transfer reference number will go for review.
These are reviewed by individuals during Mon-Fri office hours - i.e., guns gone to review this weekend will not be approved before Monday at the earliest.