For Diesel did you retain a CV board? As John said if the bulb is directly plugged into PS-1 board, or a socket on the DCRU they are 1.5V. But MTH used 6V bulbs if driven off CV board even if HL was triggered by PS-1 board through the CV board.
Best to test with 1.5V battery. If bright 1.5. If dim 6V. If off track power they are 18V. G