There's one more tag in the vim session file: after $argadd, after edit, after balt, there are 'badd' specifications. If there are 100 $argadd, then I found 99 'badd'. Since I'm looking for those few that I 'selected' by leaving a buffer/window open/active, 'badd' didfn't help either. The 'edit' single spec mentions a file that also appears in 'balt' list, BTW.
I can only think that based on 'balt' specs, open tabs/windows/buffers are re-opened, then there must be some sense of 'logical operations being applied' which then changes what files actually appear in the open GVim editor window.
What got me across the finish line was good old 'redirect' - along with 'ls' or 'history' - with option 'a', you only get a list of active/open buffers.
So: in my .vimrc I mapped Alt-S thusly:
map ó :set nomore<CR>:redir >> /tmp/chosen-files<CR>:ls a <CR>:redir END <CR>:set more<CR>-
This command, I used, not once per file but once per GVim window (per 'session', if you will) - and in the end had that list of filenames in /tmp/chosen-files.
Bob's your uncle. Hope this helps someone else in the future.
[Posted from browser on groups web site since this group does not send my own posts back to me - resulting in horrible confusion in distant past. Sorry about that.]
/Bill