Comment # 10 on bug 906589 from grant k

> NOTE the change: pubring.gpg -> pubring.kbx

exploring that change -- incl 'why' and what effect is has on this ^^^ issue
...

reading

  https://www.gnupg.org/documentation/manuals/gnupg/kbxutil.html

"...
11.1.1 Scrutinizing a keybox file

A keybox is a file format used to store public keys along with meta information
and indices. The commonly used one is the file pubring.kbx in the .gnupg
directory. It contains all X.509 certificates as well as OpenPGP keys[1].

...

Footnotes

[1] Well, OpenPGP keys are not implemented, gpg still used the keyring file
pubring.gpg
..."

I'm simply confused by that info.  Is/isn't pubring.gpg expected/required -- by
GnuPG &.or by zypper?  And is the apparent lack of a pubring.gpg, with only a
pubring.kbx present, a problem?