Do-It-Yourself PDP-11 help!

[Tony Duell]

> a Fourth Generation Systems memory board, with 18 41256s on it (512kwords?)

No. 256K 18 bit locations (i.e. 16 bit words with parity for each byte). 
So 256kW

> The little front panel has a key switch that switches the mains on and 
> off, a reset button with a "DC ok" LED, a Run/Halt switch which is 
> broken, and a three-position switch marked "L.T.C. on/off".  With this 
> switch up, the LED above it lights up.  "Local Terminal Console" is my 
> best guess so far.

Almost certainly 'Line Time Clock'. The mains-frequency interrupt signal. 
For some unknown reason this was controlled by software on Unibus systems 
(a bit in an I/O device register), but by a swtich on Q-bus machines.

-tony