C-mannosyltransferase that mediates C-mannosylation of tryptophan residues on target proteins. The reaction occurs on the luminal side of the endoplasmic reticulum and involves the transfer of a mannose unit from a dolichylphosphate mannose (Dol-P-Man) donor to an acceptor protein containing a WxxW or WxxC consensus sequence (PubMed:26764097, PubMed:29405629). C-mannosylates RSPO1, a Wnt signaling regulator, preferentially at the first Trp residue in the sequence WxxW (PubMed:26764097, PubMed:29405629). C-mannosylates the netrin receptor UNC5A, preferentially at the third tryptophan of WxxWxxWxxC sequence (By similarity). {ECO:0000250|UniProtKB:Q71B07, ECO:0000269|PubMed:26764097, ECO:0000269|PubMed:29405629}. [Isoform 2]: Has no C-mannosyltransferase activity. . This is the function of Ensembl gene identifier ENSG00000178904 (DPY19L3, dpy-19 like C-mannosyltransferase 3).