This gene/protein appears to have been described exclusively as an isozyme / electrophoretic mobility assay of a protein.
See for example this PepC mutant page:
The Pepc locus controls electrophoretic mobility of the enzyme peptidase C of red blood cells. This LT/J congenic strain carries the Pepca allele from C57BL/6 that determines a slow anodally migrating electrophoretic band.
The relevant publications describing mutants are from the 70s and 80s, e.g. here, when there were no genomic resources for mice.
However, linkage mapping of electrophoretic variants could be used to establish linkage to loci on chromosome 1, apparently.
Very likely PepC is encoded by some annotated gene on chr1, but no one knows which one exactly because they don't really know much about the protein other than its electrophoretic variability. But that's just a guess by me.
When the databases were set up, they had to accommodate 1) mouse mutant strains, and 2) published articles referencing the protein, so they did their best.
However, no one ever followed up to link everything together, so we just have these orphan entries.