[Hammarlund] SP-600JX-17 Mounting screw size?

[Bill]

Help...
Why can't I find the right screw size, to mount the SP-600JX-17 in an 
original cabinet? Fine thread, course thread??? I could only find a 1/4 
" screw in 1/2" and 3/4" lengths in 10-32 or fine thread. Is there a 3/16th"

I began to think maybe its through bolted, but no way can I get my hands 
in there...maybe there's a tool?

So help! What is screw size and thread? I've had these two for years but 
never had the screws to fasten into the cabinets. They've always been in 
use but sitting in the same place. hi

73 in advance,
Bill W2CQ