OK so I have ordered new switches and in the meantime had a closer look at these


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So this might be surprising but looks like Boxford have chosen to wire these lamps like xmas tree lamps ie in series .
So our green supply wire seems to ring around the switch commons then on to the two lamps in series. How do I add the forward voltages in this configuration??

Now as Inee points out from the Boxford wiring diagram the input voltage is 10v and there is no sign of a resistor . Is it possible that the lamp resistances stack up so that no external resistor is needed . and how would that be affected if i chose to supply 12v?