I've had the same some time ago with the backside of my Blackheart Combo.
It's just kind of glue from the tolex.
Put in some bigger screws to get it with pliers and pull it out
My latest aquisition finally reached my door
Takamine TSP 178 ACK N
What a fantastic guitar!
Feels close to a SemiHollow
Sounds way bigger than it looks
The perfect mix of SemiHollow-FullHollow-E-Acoustic
It is a keeper!
with friends....