That's a great looking amp. I have seen them with 6 channels but not 5. It would really be interesting to see the preamp internals by unscrewing that front panel. Is it a transistor based circuit like the 6 channel ones? Also, the reverb intrigues me. There must be a spring unit in there somewhere, ...

The screws are actually 0BA this is British association screw sizes that were used in lots of British equipment. They should be available on the bay or somewhere.