Most likely a factory error, either in the casting date tag or in the setup of the gang-stamp holder.
The casting date tag was brass, with the individual characters soldered to the tag, then attached with screws to the pattern; when the (sand) mold was created from the pattern, the characters appeared as raised. There were multiple patterns for the same casting (each of which had their own pattern number for traceability), and they might have missed a date change on one of the multiple patterns.
Photos below of typical casting number/date/pattern number tags, and of a Tonawanda pad-stamp gang-holder.
