Maybe the following is helpful for some of you:
SciReptor - our group's single-cell Ig analysis toolkit - ships with a manually curated C57BL/6J Ig germline database (DB). This DB is based on the NCBI mouse genome assemblies 37 and 38  and has been extensively cross-checked for accurate segment boundaries using single-cell transcript data. It uses the IgBLAST/NCBI segment nomenclature, which refers to Johnston et al. and Brekke et al.. We have now released a new version of this library.
The data is now provided as CSVs containing the start-end positions of the segments  relative to the NCBIm38 assembly. We hope that by using this format the DB versioning will become more simple and transparent compared to raw FASTA files.
 The NCBIm38 assembly is now called GRCm38 by Ensembl, but the segment locations are still identical. We will change the designation in the next release.
 For V segments, only the positions of exon 2 are provided since exon 1 always encodes the leader peptide. Exon 1 positions are available if the exon is productive.