Mappings between coding systems

Questions arising from the work on coding system releases:

We have two mapping apps, which map ctv3 to snomed, and bnf to dmd. Both of these import data from a single mapping file; although it seems unlikely that there will ever be any more updates for ctv3/snomed.
https://nhs-digital.citizenspace.com/standards-assurance/mapping-tables-read-v2-snomed/

BNF to dm+d mappings are released as excel spreadsheets here, however, there isn't any indication of which version of each coding system they relate to.

Currently, (or at least, as of #1415) the converters assume that we're always talking about the latest coding system release. Given that there aren't likely to be more ctv3 to snomed mappings, we can probably limit concerns to BNF to dm+d mappings.

1. Can we identify a release of dm+d and BNF from the mapping files? Or can we at least identify whether a mapping release is incompatible with any particular dm+d or BNF release (i.e. because there are codes in the mapping that aren't in a dm+d/BNF release)
2. Should we also be keeping historic releases of the mapping?
3. The mapping is used for a dm+d download of a BNF codelist; if we've been able to identify mappings that are compatible with releases of BNF/dm+d, do we want users to be able to select the dm+d release that's used for downloading?
4. Do the mappings change? If we have the same code mapped differently in 2 mapping files, but the mapping files are compatible with multiple versions of the coding systems, which mapping do we choose?