69 Twin Reverb - Needs some guidance on speaker fasteners

[Stevie B]

I am fixing a 1969 Twin Reverb for a friend.
I had a few issues - reverb out, two prong power cord complete with death cap, slight hum.
But...

[cablercdd1]

I've got a JBL EON 518S Sub that someone asked me to look at. The switching power supply appears to work well and the preamp signal led lights up when I apply...