My UNF bolts that fit (from a collection) have a 5/16" head, I do not have the original bolts - this "packer" was in my "box of bits" with the gear lever and original sandwich plates. The replacement and new components, (as I opted to replace this assembly comprising rose joint and sandwich plates are tapped for 5mm). Note: You will recall that only one of the sandwich plates is tapped - the other has clearance holes.
I agree - it could only go "on top" - but this would be between the sandwich plates containing the rose joint and the chassis and the only reason I could see that you would want to do this would be to raise the fulcrum point of the gear lever/1st section of gear change shaft inside the tunnel.
I think it is a PO mystery and I will leave it out of the install.