Many thanks to you all for your helpful responses. I am going to give this a go. I have already noticed that there is 240v flying around particularly at the front in the coin mech area. Interesting that the person I bought it from has used it for many years with a two core mains wire and NO EARTH!
That's the way they all were manufactured and supplied pretty well right the way up to the arrival of solid state games - Electrical safety standards were very different until not that long ago.
I've often had a mild tingle from older games, but in 55 years of involvement with the hobby have fortunately never seen or heard a report of a player outside a closed machine being killed or seriously injured by electric shock from one
(no doubt someone will be able to point me to one now I've said that!)
There is typically 30 - 35v on the lockdown bar/siderails of an unearthed game, but because the two cores are not differentiated on the original wiring it is possible for two games side by side to be wired oppositely creating a 70v potential difference between them
Gottlieb are worse on electrical safety than Williams - their coin door has mains voltage switches on it which you push closed with a metal start button.
The only thing that protects you is a thin strip of insulating "fish paper" which, over the years, can wear and develop a small hole.
I've had a nasty shock from one of these and would always recommend fitting a new fish paper strip (or an alternative method of starting the game).
See section 3k of the Pinrepair document I linked in the earlier post for more info and pictures on this
If you are fitting a three core cable and internal earthing the manufacturers method, and the one you will see referred to online, is to attach the outgoing earth wire to the transformer frame (via the securing nuts) and then radiate out from there to the exposed metal parts.
I had a significant problem with this with one of my machines - the outgoing connection failed and there was significant voltage on the transformer frame (I'm told this is because old transformers "leak" voltage) which was then fed to every exposed metal part via the earthing I had put in, making the machine more dangerous than in it's original form.
Since that experience, and in conversation with knowledgeable others, I now attach the outgoing earth wire to a central distribution point screwed into the wood mechanism board on the base of the machine, and don't put in any connection with the transformer frame (for which I have not seen any logic for in the first place)
For our shows we insulate all exposed metal parts, including the legs (via the leg plates)
Please note that I have no electrical training and would defer to the more informed opinion of someone who has if they should contradict me
For the record, although Giacomo at Pinball Creative produced some fabulously beautiful restorations (at equally fabulous prices), on a couple of occasions I had reason to doubt the depth of his knowledge on the electrical side so I would not go solely by any advice he may give.