Why do I have V8 and v10 folders under Ventura Mail

I recently migrated my 2015 MBP running Monterey to a new Mac Mini running Ventura.

I notice that under my ~/Library/Mail directory that I have both V8 an V10 folders. Since Ventura Mail is V10, I am trying to figure out what the very large V8 folder is for. When I look in ~/Library/Mail I see:

➜ Mail ls -l

total 8

drwxr-xr-x@  2 jrh staff   64 Oct 10 2016 Bundles

drwxr-xr-x@ 1110 jrh staff 35520 Oct 10 2016 MPT

drwxr-xr-x@  2 jrh staff   64 Sep 28 2021 Metadata

-rw-r--r--@  1 jrh staff  1427 Jan 28 18:07 PersistenceInfo.plist

drwxr-xr-x@  20 jrh staff  640 Apr 7 2022 V10

drwxr-xr-x@  11 jrh staff  352 Sep 28 2021 V8

Then when I look at the size of these folders I see:

➜ Mail ** -sh V*

 69G V10

 39G V8

How can I find out what is in the 39G v8 folder. When I look thru it I see 5 subdirectories that may (or may not) correspond to my 5 email accounts. Digging deeper into one I see several mbox files that also exist in (in this case) my iCloud mail account.

➜  ls V8/980D8933-D666-4D3B-9D2D-7A8A254A7BFA

Apple Mail To Do.mbox       Junk.mbox

Archive.mbox           Magnum.mbox

Cameras.mbox           New Folder.mbox

Cottage.mbox           Notes.mbox

Deleted Messages.mbox       Political Crap.mbox

Drafts.mbox            Sent Messages (.Mac Account).mbox

Family.mbox            Sent Messages.mbox

Financial.mbox          ffmpeg.mbox

INBOX.mbox

I see most of these mailboxes in Mail under my iCloud account and they appear to be fully populated.

Is mail using both the V8 and V10 mbox structure or is this 39G just cruft left over from a past conversion?