There should be different lugs with different voltages on the transformer, not sure about 230 V but could be 220 and 240.
Solder the lug on 240 then, measure the outcoming voltage on 24 V, if it's to low solder the 24 V line on the high tap.

not great sorry. hes wrong.

the williams EM's have different transformers and motors for 110v 60hz and 230v 50hz

evidence ( example stardust)
http://www.planetarypinball.com/reference/partsmanuals/WMS_Parts_1977-1978/index.html#/138/zoomed

Looking at the place of origin: Reykjavik I assumed that it would be a 50 Hz.
That is the standard as I learned on Google, so if it isn't a imported machine from the U.S. it's likely to be a 15B-6773 transformer which can be set on:

105 V
117 V
130 V
160 V
205 V
225 V
240 V

Advised on 220 V is the 225 V lug.

interesting but still wrong.
as he says its a 110v machine.
it will be a 15a-6771 and he needs to buy a 15a-6782-1

the schematic agrees
http://mirror2.ipdb.org/files/1072/Williams_1976_Grand_Prix_Schematic_Diagram_paginated_includes_correction_to_L_R_Kickers.pdf

the parts manual agrees. (they forgot to add grand prix it seems but it agrees for every other machine around that time)

here you go.. they didnt forgot to add grand prix, its just not on the pps website version.

Screen Shot 2015-02-22 at 5.20.22 pm.png

Well explained, I'll keep out of this thread.
Wouldn't like to give the wrong information or advise

no please stay .. we are all friends .. stay, learn, help, play .. have another beer

Can't you get an external transformer that will reduce your line voltage to 110V? The frequency is another issue; the motor will run slower at 50hz, but the game will still work. I've had games here in the US that were imported from Europe with a 50hz motor in them. The motor runs faster here with 60hz, and sometimes the score reels can't keep up when counting 50 or 500 points, but otherwise the game works.