From my notebook...
...dimensions and cutting notes (on the small lathe) for signal lamps, based on GWR drawings reproduced in Adrian Vaughan's 'Great Western Signalling". The lamps are bored out 3mm to house a 3mm water clear LED. Although not common, I sourced some flangeless LEDs (Toby Electronics - Toby.co.uk) which will fit right into the lamp case, leaving only the clear base of the LED to be painted and stop light spillage. The lamp is cross drilled to take the front an