@C-Val,
All I know for sure is what I have in my '84 C-Dory. All balsa in the transom and standard full 'o void exterior CDX plywood in the sole - nope, no marine-grade stuff there... There was no plywood in my boat's transom. YBMV...
In regard to your boat, an electrostatic moisture-meter can tell you a lot once you get the hang of interpreting the message. In a dry transom with both balsa and plywood, you'll get a higher reading over the plywood due to a higher density of material. Water will do the same thing. It gets a bit complicated, but with a little time and comprehension of what may cause variances in the meter readings you'll get the hang of it. I'm no rocket-scientist, but after I mapped out the readings on the fiberglass, I removed it and found that those readings accurately reflected the character of the core below.
I'm hoping to crank my project back up in the very near future. The exterior hull laminations were almost done late last summer when a series of physical maladies conspired to keep me out of the shop. Feeling progressively better as of late, so look out... Of course, I have also found real work at a day job doing GIS/mapping, so that cuts into the boat project, but it's nice to have funds for the gelcoat and possible repower too.
It's all fun...