I have finally resolved my problem with the 001 field (new table in the Koha db and new Perl script) and put it into our production server.

However, I *may* have discovered a bug and would appreciate if someone could confirm whether it is intrinsic to 3.8.5 or to our specific installation [both my sandbox and production server exhibit the same behaviour, but use the same db.] If it is a bug, I will be happy to supply my solution if there is not a more elegant fix.

Basically, in the MARC Frameworks Constraints / Other Options / Plugin, it does not matter what plugin script name is used, Koha continues to use callnumber.pl regardless. However, at item level,  952$o ("full call number") will accept and run a differently named script, e.g. our_callnumber.pl

I have checked the relevant row in marc_subfield_structure as: ([] added for clarity)

1 @ [control field] [control field] 0 0 [blank] 0 [blank] [blank] controlnumber.pl 0 0 [blank] \N [blank] [blank] 9999

and do not have a working script called controlnumber.pl -- I have an empty file and just added it to 001 / Constraints / Other Options / Plugin to demonstrate that callnumber.pl is in fact being used - modified for our use -- and if I remove or rename callnumber.pl the system fails.  I haven't found anywhere that callnumber.pl might be "hardcoded" ... perhaps somewhere, I just haven't had time to go beyond a quick grep.

Best - Paul